(front) Albums: link to artist/albums + Play count

Improve ngIf part
Use a script in ELS query to sort on avg play count

dashboard/src/app/els-album.service.ts

@@ -14,7 +14,16 @@ export class ElsAlbumService extends ElsService {
       'Avg Bit Rate': {
         'order': 'asc'
       }
-    } ],
+    }, {
+      '_script' : {
+          'type' : 'number',
+          'script' : {
+              'lang': 'painless',
+              'source': 'doc[\'Play Count\'].value / doc[\'Track Count\'].value'
+          },
+          'order' : 'desc'
+      }
+  } ],
     'size': ElsService.DEFAULT_SIZE
   }