Dashboard: Support configuring default timezone via config file (#27404)

Add a default timezone that the administrator can set in the settings. 
This setting is be used as default for the users timezone preference.
Can be used when creating Grafana instances without administrator 
intervention, in order to give user the correct default timezone.

Fixes #25654

pkg/setting/date_formats.go

@@ -1,9 +1,16 @@
 package setting
 
+import (
+	"time"
+
+	"gopkg.in/ini.v1"
+)
+
 type DateFormats struct {
 	FullDate         string              `json:"fullDate"`
 	UseBrowserLocale bool                `json:"useBrowserLocale"`
 	Interval         DateFormatIntervals `json:"interval"`
+	DefaultTimezone  string              `json:"defaultTimezone"`
 }
 
 type DateFormatIntervals struct {
@@ -15,6 +22,23 @@ type DateFormatIntervals struct {
 	Year   string `json:"year"`
 }
 
+const LocalBrowserTimezone = "browser"
+
+func valueAsTimezone(section *ini.Section, keyName string, defaultValue string) (string, error) {
+	timezone := section.Key(keyName).MustString(defaultValue)
+
+	if timezone == LocalBrowserTimezone {
+		return LocalBrowserTimezone, nil
+	}
+
+	location, err := time.LoadLocation(timezone)
+	if err != nil {
+		return LocalBrowserTimezone, err
+	}
+
+	return location.String(), nil
+}
+
 func (cfg *Cfg) readDateFormats() {
 	dateFormats := cfg.Raw.Section("date_formats")
 	cfg.DateFormats.FullDate = valueAsString(dateFormats, "full_date", "YYYY-MM-DD HH:mm:ss")
@@ -25,4 +49,10 @@ func (cfg *Cfg) readDateFormats() {
 	cfg.DateFormats.Interval.Month = valueAsString(dateFormats, "interval_month", "YYYY-MM")
 	cfg.DateFormats.Interval.Year = "YYYY"
 	cfg.DateFormats.UseBrowserLocale = dateFormats.Key("date_format_use_browser_locale").MustBool(false)
+
+	timezone, err := valueAsTimezone(dateFormats, "default_timezone", LocalBrowserTimezone)
+	if err != nil {
+		cfg.Logger.Warn("Unknown timezone as default_timezone", "err", err)
+	}
+	cfg.DateFormats.DefaultTimezone = timezone
 }