Fix bug preventing setting room power levels

- don't use refs, use onChange of PowerSelector
 - store power levels as state in the RoomSetting component

test/components/views/rooms/RoomSettings-test.js

@@ -1,8 +1,6 @@
 import React from 'react';
-import ReactTestUtils from 'react-addons-test-utils';
 import ReactDOM from 'react-dom';
 import expect, {createSpy} from 'expect';
-import sinon from 'sinon';
 import Promise from 'bluebird';
 import * as testUtils from '../../../test-utils';
 import sdk from 'matrix-react-sdk';