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/MessageFactory.cs

@@ -59,6 +59,22 @@ namespace API.SignalR
             };
         }
 
+        public static SignalRMessage ScanLibraryProgressEvent(int libraryId, int progress, string seriesName)
+        {
+            return new SignalRMessage()
+            {
+                Name = SignalREvents.ScanLibrary,
+                Body = new
+                {
+                    LibraryId = libraryId,
+                    Progress = progress,
+                    SeriesName = seriesName
+                }
+            };
+        }
+
+
+
         public static SignalRMessage RefreshMetadataEvent(int libraryId, int seriesId)
         {
             return new SignalRMessage()