feat: add configurable gravatar-compatible base URL (#2083)

This adds the ability to set a base URL for a Gravatar-compatible avatar
service (Gravatar itself, or Libravatar, for instance). The default will
be www.gravatar.com, so nothing will change from current behaviour unless
the user explicitly configures another URL.

Resolves #2082

pkg/config/config.go

@@ -187,6 +187,7 @@ const (
 	CorsMaxAge  Key = `cors.maxage`
 
 	AvatarGravaterExpiration Key = `avatar.gravatarexpiration`
+	AvatarGravatarBaseURL    Key = `avatar.gravatarbaseurl`
 
 	BackgroundsEnabled               Key = `backgrounds.enabled`
 	BackgroundsUploadEnabled         Key = `backgrounds.providers.upload.enabled`
@@ -456,6 +457,7 @@ func InitDefaultConfig() {
 	MigrationMicrosoftTodoEnable.setDefault(false)
 	// Avatar
 	AvatarGravaterExpiration.setDefault(3600)
+	AvatarGravatarBaseURL.setDefault("https://www.gravatar.com")
 	// Project Backgrounds
 	BackgroundsEnabled.setDefault(true)
 	BackgroundsUploadEnabled.setDefault(true)
@@ -605,6 +607,12 @@ func InitConfig() {
 
 	readConfigValuesFromFiles()
 
+	if _, err := url.ParseRequestURI(AvatarGravatarBaseURL.GetString()); err != nil {
+		log.Fatalf("Could not parse gravatarbaseurl: %s", err)
+	}
+
+	AvatarGravatarBaseURL.Set(strings.TrimRight(AvatarGravatarBaseURL.GetString(), "/"))
+
 	if RateLimitStore.GetString() == "keyvalue" {
 		RateLimitStore.Set(KeyvalueType.GetString())
 	}