Warn on Refresh Metadata (#610)

* Warn the user about the dangers of refresh metadata and promote them to use a scan instead.

* Removed presence hub and moved it over to message hub.

When a library scan is in progress, now a spinner will show on manage libraries page.

* Code cleanup

API/Startup.cs

@@ -147,7 +147,6 @@ namespace API
             {
                 endpoints.MapControllers();
                 endpoints.MapHub<MessageHub>("hubs/messages");
-                endpoints.MapHub<PresenceHub>("hubs/presence");
                 endpoints.MapHangfireDashboard();
                 endpoints.MapFallbackToController("Index", "Fallback");
             });