Implement TagPanel (or LeftLeftPanel) for group filtering

This allows for filtering of the RoomList by group. When a group is selected, the room list will show:
 - Rooms in the group
 - Direct messages with members in the group

A button at the bottom of the TagPanel allows for creating new groups, which will appear in the panel following creation.

src/stores/GroupStoreCache.js

@@ -28,7 +28,6 @@ class GroupStoreCache {
             // referencing it.
             this.groupStore = new GroupStore(groupId);
         }
-        this.groupStore._fetchSummary();
         return this.groupStore;
     }
 }