Fix goconst issues

See,
$ gometalinter --vendor --disable-all --enable=goconst --disable=gotype --deadline=6m ./...
build.go:113:15⚠️ 2 other occurrence(s) of "linux" found in: build.go:119:15 build.go:491:34 (goconst)
build.go:119:15⚠️ 2 other occurrence(s) of "linux" found in: build.go:113:15 build.go:491:34 (goconst)
build.go:491:34⚠️ 2 other occurrence(s) of "linux" found in: build.go:113:15 build.go:119:15 (goconst)
build.go:381:21⚠️ 2 other occurrence(s) of "windows" found in: build.go:423:13 build.go:487:13 (goconst)
build.go:423:13⚠️ 2 other occurrence(s) of "windows" found in: build.go:381:21 build.go:487:13 (goconst)
build.go:487:13⚠️ 2 other occurrence(s) of "windows" found in: build.go:381:21 build.go:423:13 (goconst)
pkg/api/dashboard.go:67:22⚠️ 5 other occurrence(s) of "Anonymous" found in: pkg/api/dashboard.go:67:35 pkg/api/dashboard.go:131:10 pkg/api/dashboard.go:406:13 pkg/api/folder.go:98:22 pkg/api/folder.go:98:35 (goconst)
pkg/api/dashboard.go:67:35⚠️ 5 other occurrence(s) of "Anonymous" found in: pkg/api/dashboard.go:67:22 pkg/api/dashboard.go:131:10 pkg/api/dashboard.go:406:13 pkg/api/folder.go:98:22 pkg/api/folder.go:98:35 (goconst)
pkg/api/dashboard.go:131:10⚠️ 5 other occurrence(s) of "Anonymous" found in: pkg/api/dashboard.go:67:22 pkg/api/dashboard.go:67:35 pkg/api/dashboard.go:406:13 pkg/api/folder.go:98:22 pkg/api/folder.go:98:35 (goconst)
pkg/api/dashboard.go:406:13⚠️ 5 other occurrence(s) of "Anonymous" found in: pkg/api/dashboard.go:67:22 pkg/api/dashboard.go:67:35 pkg/api/dashboard.go:131:10 pkg/api/folder.go:98:22 pkg/api/folder.go:98:35 (goconst)
pkg/api/folder.go:98:22⚠️ 5 other occurrence(s) of "Anonymous" found in: pkg/api/dashboard.go:67:22 pkg/api/dashboard.go:67:35 pkg/api/dashboard.go:131:10 pkg/api/dashboard.go:406:13 pkg/api/folder.go:98:35 (goconst)
pkg/api/folder.go:98:35⚠️ 5 other occurrence(s) of "Anonymous" found in: pkg/api/dashboard.go:67:22 pkg/api/dashboard.go:67:35 pkg/api/dashboard.go:131:10 pkg/api/dashboard.go:406:13 pkg/api/folder.go:98:22 (goconst)
pkg/api/index.go:63:47⚠️ 2 other occurrence(s) of "light" found
in: pkg/api/index.go:91:22 pkg/api/index.go:93:16 (goconst)
pkg/api/index.go:91:22⚠️ 2 other occurrence(s) of "light" found in: pkg/api/index.go:63:47 pkg/api/index.go:93:16 (goconst)
pkg/api/index.go:93:16⚠️ 2 other occurrence(s) of "light" found in: pkg/api/index.go:63:47 pkg/api/index.go:91:22 (goconst)
pkg/components/null/float.go:71:25⚠️ 2 other occurrence(s) of "null" found in: pkg/components/null/float.go:103:10 pkg/components/null/float.go:112:10 (goconst)
pkg/components/null/float.go:103:10⚠️ 2 other occurrence(s) of "null" found in: pkg/components/null/float.go:71:25 pkg/components/null/float.go:112:10 (goconst)
pkg/components/null/float.go:112:10⚠️ 2 other occurrence(s) of "null" found in: pkg/components/null/float.go:71:25 pkg/components/null/float.go:103:10 (goconst)
pkg/services/alerting/notifiers/pagerduty.go:79:16⚠️ 2 other occurrence(s) of "Triggered metrics:\n\n" found in: pkg/services/alerting/notifiers/kafka.go:64:16 pkg/services/alerting/notifiers/opsgenie.go:98:16 (goconst)
pkg/services/alerting/notifiers/kafka.go:64:16⚠️ 2 other occurrence(s) of "Triggered metrics:\n\n" found in: pkg/services/alerting/notifiers/pagerduty.go:79:16 pkg/services/alerting/notifiers/opsgenie.go:98:16 (goconst)
pkg/services/alerting/notifiers/opsgenie.go:98:16⚠️ 2 other occurrence(s) of "Triggered metrics:\n\n" found in: pkg/services/alerting/notifiers/pagerduty.go:79:16 pkg/services/alerting/notifiers/kafka.go:64:16 (goconst)
pkg/social/social.go:85:11⚠️ 2 other occurrence(s) of "grafana_com" found in: pkg/social/social.go:162:14 pkg/social/social.go:197:11 (goconst)
pkg/social/social.go:162:14⚠️ 2 other occurrence(s) of "grafana_com" found in: pkg/social/social.go:85:11 pkg/social/social.go:197:11 (goconst)
pkg/social/social.go:197:11⚠️ 2 other occurrence(s) of "grafana_com" found in: pkg/social/social.go:85:11 pkg/social/social.go:162:14 (goconst)
pkg/tsdb/elasticsearch/time_series_query.go:92:17⚠️ 3 other occurrence(s) of "count" found in: pkg/tsdb/elasticsearch/response_parser.go:152:8 pkg/tsdb/elasticsearch/response_parser.go:167:31 pkg/tsdb/elasticsearch/response_parser.go:315:9 (goconst)
pkg/tsdb/elasticsearch/response_parser.go:152:8⚠️ 3 other occurrence(s) of "count" found in: pkg/tsdb/elasticsearch/time_series_query.go:92:17 pkg/tsdb/elasticsearch/response_parser.go:167:31 pkg/tsdb/elasticsearch/response_parser.go:315:9 (goconst)
pkg/tsdb/elasticsearch/response_parser.go:167:31⚠️ 3 other occurrence(s) of "count" found in: pkg/tsdb/elasticsearch/time_series_query.go:92:17 pkg/tsdb/elasticsearch/response_parser.go:152:8 pkg/tsdb/elasticsearch/response_parser.go:315:9 (goconst)
pkg/tsdb/elasticsearch/response_parser.go:315:9⚠️ 3 other occurrence(s) of "count" found in: pkg/tsdb/elasticsearch/time_series_query.go:92:17 pkg/tsdb/elasticsearch/response_parser.go:152:8 pkg/tsdb/elasticsearch/response_parser.go:167:31 (goconst)
pkg/tsdb/elasticsearch/time_series_query.go:78:9⚠️ 2 other occurrence(s) of "date_histogram" found in: pkg/tsdb/elasticsearch/response_parser.go:84:22 pkg/tsdb/elasticsearch/response_parser.go:369:24 (goconst)
pkg/tsdb/elasticsearch/response_parser.go:84:22⚠️ 2 other occurrence(s) of "date_histogram" found in: pkg/tsdb/elasticsearch/time_series_query.go:78:9 pkg/tsdb/elasticsearch/response_parser.go:369:24 (goconst)
pkg/tsdb/elasticsearch/response_parser.go:369:24⚠️ 2 other occurrence(s) of "date_histogram" found in: pkg/tsdb/elasticsearch/time_series_query.go:78:9 pkg/tsdb/elasticsearch/response_parser.go:84:22 (goconst)

pkg/api/index.go

@@ -11,6 +11,12 @@ import (
 	"github.com/grafana/grafana/pkg/setting"
 )
 
+const (
+	// Themes
+	lightName = "light"
+	darkName  = "dark"
+)
+
 func setIndexViewData(c *m.ReqContext) (*dtos.IndexViewData, error) {
 	settings, err := getFrontendSettingsMap(c)
 	if err != nil {
@@ -60,7 +66,7 @@ func setIndexViewData(c *m.ReqContext) (*dtos.IndexViewData, error) {
 			OrgRole:                    c.OrgRole,
 			GravatarUrl:                dtos.GetGravatarUrl(c.Email),
 			IsGrafanaAdmin:             c.IsGrafanaAdmin,
-			LightTheme:                 prefs.Theme == "light",
+			LightTheme:                 prefs.Theme == lightName,
 			Timezone:                   prefs.Timezone,
 			Locale:                     locale,
 			HelpFlags1:                 c.HelpFlags1,
@@ -88,12 +94,12 @@ func setIndexViewData(c *m.ReqContext) (*dtos.IndexViewData, error) {
 	}
 
 	themeURLParam := c.Query("theme")
-	if themeURLParam == "light" {
+	if themeURLParam == lightName {
 		data.User.LightTheme = true
-		data.Theme = "light"
-	} else if themeURLParam == "dark" {
+		data.Theme = lightName
+	} else if themeURLParam == darkName {
 		data.User.LightTheme = false
-		data.Theme = "dark"
+		data.Theme = darkName
 	}
 
 	if hasEditPermissionInFoldersQuery.Result {