Implement "Add room to group" feature

This step needs to be taken before adding a room to a group summary

src/skins/vector/css/vector-web/structures/_RightPanel.scss

@@ -78,6 +78,7 @@ limitations under the License.
 
 .mx_RightPanel .mx_MemberList,
 .mx_RightPanel .mx_MemberInfo,
+.mx_RightPanel .mx_GroupRoomList,
 .mx_RightPanel_blank {
     order: 2;
     flex: 1 1 0;