Hide the user menu in the GDM greeter lock screen

https://bugzilla.gnome.org/show_bug.cgi?id=683705

js/ui/sessionMode.js

@@ -48,6 +48,7 @@ const _modes = {
 
     'lock-screen': {
         isLocked: true,
+        isGreeter: undefined,
         unlockDialog: undefined,
         components: ['networkAgent', 'polkitAgent', 'telepathyClient'],
         panel: {

js/ui/userMenu.js

@@ -577,6 +577,8 @@ const UserMenuButton = new Lang.Class({
     },
 
     _sessionUpdated: function() {
+        this.actor.visible = !Main.sessionMode.isGreeter;
+
         let allowSettings = Main.sessionMode.allowSettings;
         this._statusChooser.setSensitive(allowSettings);
         this._systemSettings.visible = allowSettings;