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/server/wire.go

@@ -7,6 +7,7 @@ import (
 	"github.com/google/wire"
 	sdkhttpclient "github.com/grafana/grafana-plugin-sdk-go/backend/httpclient"
 	"github.com/grafana/grafana/pkg/api"
+	"github.com/grafana/grafana/pkg/api/avatar"
 	"github.com/grafana/grafana/pkg/api/routing"
 	"github.com/grafana/grafana/pkg/bus"
 	"github.com/grafana/grafana/pkg/expr"
@@ -232,6 +233,7 @@ var wireBasicSet = wire.NewSet(
 	wire.Bind(new(alerting.DashAlertExtractor), new(*alerting.DashAlertExtractorService)),
 	comments.ProvideService,
 	guardian.ProvideService,
+	avatar.ProvideAvatarCacheServer,
 	authproxy.ProvideAuthProxy,
 )