Don't re-render matrixchat unnecessarily

...on room switch. We were setting most of the state in viewRoom,
but getting the current room ID from the RoomViewStore, but this
meant we did one setState from the RoomViewStore updating,
re-rendered and then setState again in viewRoom causing another
render. This just sets the room ID in viewRoom.

src/components/structures/MatrixChat.js

@@ -38,7 +38,6 @@ import linkifyMatrix from "../../linkify-matrix";
 import * as Lifecycle from '../../Lifecycle';
 // LifecycleStore is not used but does listen to and dispatch actions
 require('../../stores/LifecycleStore');
-import RoomViewStore from '../../stores/RoomViewStore';
 import PageTypes from '../../PageTypes';
 
 import createRoom from "../../createRoom";
@@ -214,9 +213,6 @@ module.exports = React.createClass({
     componentWillMount: function() {
         SdkConfig.put(this.props.config);
 
-        this._roomViewStoreToken = RoomViewStore.addListener(this._onRoomViewStoreUpdated);
-        this._onRoomViewStoreUpdated();
-
         if (!UserSettingsStore.getLocalSetting('analyticsOptOut', false)) Analytics.enable();
 
         // Used by _viewRoom before getting state from sync
@@ -587,10 +583,6 @@ module.exports = React.createClass({
         }
     },
 
-    _onRoomViewStoreUpdated: function() {
-        this.setState({ currentRoomId: RoomViewStore.getRoomId() });
-    },
-
     _setPage: function(pageType) {
         this.setState({
             page_type: pageType,
@@ -677,6 +669,7 @@ module.exports = React.createClass({
         this.focusComposer = true;
 
         const newState = {
+            currentRoomId: roomInfo.room_id || null,
             page_type: PageTypes.RoomView,
             thirdPartyInvite: roomInfo.third_party_invite,
             roomOobData: roomInfo.oob_data,