Factor out logic from EnableNotificationsButton(!) and reuse MatrixToolbar.

Added notification logic to Notifier; dispatch notifier_enabled when toggled
so the toolbar can be shown/hidden and the button text can be kept in sync.
Add MatrixToolbar back into MatrixChat for notification nagging.

skins/base/views/molecules/MatrixToolbar.js

@@ -32,8 +32,7 @@ module.exports = React.createClass({
     render: function() {
         return (
             <div className="mx_MatrixToolbar">
-                <LogoutButton />
-                <EnableNotificationsButton />
+                You are not receiving desktop notifications. <EnableNotificationsButton />
             </div>
         );
     }