message: add in scrollto button
diff: /*
  Copyright 2016 OpenMarket Ltd
  +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.
  @@ -32,7 +33,10 @@ module.exports = React.createClass({
  <div className="mx_TopUnreadMessagesBar">
  <div className="mx_TopUnreadMessagesBar_scrollUp"
  onClick={this.props.onScrollUpClick}>
  - Jump to first unread message. <span style={{ textDecoration: 'underline' }} onClick={this.props.onCloseClick}>Mark all read</span>
  + <img src="img/scrollto.svg" width="24" height="24"
  + alt="Scroll to unread messages"
  + title="Scroll to unread messages"/>
  + Jump to first unread message.
  </div>
  <img className="mx_TopUnreadMessagesBar_close"
  src="img/cancel.svg" width="18" height="18"
  