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/SignalR/SignalREvents.cs

@@ -8,6 +8,7 @@
         public const string ScanLibrary = "ScanLibrary";
         public const string SeriesAdded = "SeriesAdded";
         public const string SeriesRemoved = "SeriesRemoved";
-
+        public const string ScanLibraryProgress = "ScanLibraryProgress";
+        public const string OnlineUsers = "OnlineUsers";
     }
 }