Add metric warning support (announcement bar and DM) (#14483)

* Admin. Advisory: Add warning for number of active users metric status

Co-authored-by: Catalin Tomai <catalin.tomai@mattermost.com>

model/config.go

@@ -47,6 +47,7 @@ const (
 	GENERIC_NO_CHANNEL_NOTIFICATION = "generic_no_channel"
 	GENERIC_NOTIFICATION            = "generic"
 	GENERIC_NOTIFICATION_SERVER     = "https://push-test.mattermost.com"
+	MM_SUPPORT_ADDRESS              = "support@mattermost.com"
 	FULL_NOTIFICATION               = "full"
 	ID_LOADED_NOTIFICATION          = "id_loaded"