Dashboard: Mix initials and custom gravatars in recent viewers list (#47212)

* Use Wiring to initialize Avatar Cache Server

Create AvatarCacheServer Provider function and pass it in as an
argument to HTTPServer. Also convert CacheServer to a singleton
so that we keep all cached Avatar info in one place for easier access

* Refactor avatar cache server and add 'isCustom' check

Avatar cache server needs to perform two similar fetches
back-to-back; break up functions to allow for easy reuse.
Then add handling to see if a user has a custom avatar.

* Add additional accessors so that /recents api can easily use the cache

* Minor mods to avatar server to facilitiate unit testing

* add unit tests for avatar fetching

* add error handling in case we somehow fetch gravatars while they are disabled

* linting: read error return value in unit test

* Use http package status codes

Co-authored-by: Ezequiel Victorero <evictorero@gmail.com>

* Use http package status codes

Co-authored-by: Ezequiel Victorero <evictorero@gmail.com>

* Use http package status codes

Co-authored-by: Ezequiel Victorero <evictorero@gmail.com>

* Incorporate suggestions from PR
-avoid mutating arguments
-change error handler function to private and make name more descriptive

Co-authored-by: Ezequiel Victorero <evictorero@gmail.com>

pkg/api/http_server.go

@@ -13,6 +13,7 @@ import (
 	"strings"
 	"sync"
 
+	"github.com/grafana/grafana/pkg/api/avatar"
 	"github.com/grafana/grafana/pkg/api/routing"
 	httpstatic "github.com/grafana/grafana/pkg/api/static"
 	"github.com/grafana/grafana/pkg/bus"
@@ -147,6 +148,7 @@ type HTTPServer struct {
 	AlertNotificationService     *alerting.AlertNotificationService
 	DashboardsnapshotsService    *dashboardsnapshots.Service
 	PluginSettings               *pluginSettings.Service
+	AvatarCacheServer            *avatar.AvatarCacheServer
 }
 
 type ServerOptions struct {
@@ -178,6 +180,7 @@ func ProvideHTTPServer(opts ServerOptions, cfg *setting.Cfg, routeRegister routi
 	dashboardProvisioningService dashboards.DashboardProvisioningService, folderService dashboards.FolderService,
 	datasourcePermissionsService permissions.DatasourcePermissionsService, alertNotificationService *alerting.AlertNotificationService,
 	dashboardsnapshotsService *dashboardsnapshots.Service, commentsService *comments.Service, pluginSettings *pluginSettings.Service,
+	avatarCacheServer *avatar.AvatarCacheServer,
 ) (*HTTPServer, error) {
 	web.Env = cfg.Env
 	m := web.New()
@@ -250,6 +253,7 @@ func ProvideHTTPServer(opts ServerOptions, cfg *setting.Cfg, routeRegister routi
 		DashboardsnapshotsService:    dashboardsnapshotsService,
 		PluginSettings:               pluginSettings,
 		permissionServices:           permissionsServices,
+		AvatarCacheServer:            avatarCacheServer,
 	}
 	if hs.Listener != nil {
 		hs.log.Debug("Using provided listener")