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/FilterStore.js

@@ -0,0 +1,65 @@
+/*
+Copyright 2017 Vector Creations Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+import {Store} from 'flux/utils';
+import dis from '../dispatcher';
+import Analytics from '../Analytics';
+
+const INITIAL_STATE = {
+    tags: [],
+};
+
+/**
+ * A class for storing application state for filtering via TagPanel.
+ */
+class FilterStore extends Store {
+    constructor() {
+        super(dis);
+
+        // Initialise state
+        this._state = INITIAL_STATE;
+    }
+
+    _setState(newState) {
+        this._state = Object.assign(this._state, newState);
+        this.__emitChange();
+    }
+
+    __onDispatch(payload) {
+        switch (payload.action) {
+            case 'select_tag':
+                this._setState({
+                    tags: [payload.tag],
+                });
+                Analytics.trackEvent('FilterStore', 'select_tag');
+            break;
+            case 'deselect_tags':
+                this._setState({
+                    tags: [],
+                });
+                Analytics.trackEvent('FilterStore', 'deselect_tags');
+            break;
+        }
+    }
+
+    getSelectedTags() {
+        return this._state.tags;
+    }
+}
+
+if (global.singletonFilterStore === undefined) {
+    global.singletonFilterStore = new FilterStore();
+}
+export default global.singletonFilterStore;

src/stores/GroupStoreCache.js

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