(js) Add hotkey to flag a message

UI/WebServerResources/js/Mailer/MessageController.js

@@ -104,6 +104,10 @@
 
 
     function _registerHotkeys(keys) {
+      keys.push(sgHotkeys.createHotkey({
+        key: l('hotkey_flag'),
+        callback: angular.bind(stateMessage, stateMessage.toggleFlag)
+      }));
       keys.push(sgHotkeys.createHotkey({
         key: 'backspace',
         callback: function($event) {