Modify GroupView UI

- Remove featured users/rooms
 - Add "Rooms" section to show all rooms in the group in a room-directory-esque list. This has a "+" button in "edit" mode.
 - Make the group view body scrollable

src/components/structures/ScrollPanel.js

@@ -148,6 +148,7 @@ module.exports = React.createClass({
             onFillRequest: function(backwards) { return Promise.resolve(false); },
             onUnfillRequest: function(backwards, scrollToken) {},
             onScroll: function() {},
+            onResize: function() {},
         };
     },