gdm: fix sensitivity of auth prompt when cancelling early and user list is disabled

If the user list is disabled and the user clicks cancel quickly enough
after typing their username, they can get in a state where the
auth prompt gets stuck in the insensitive state.

This is because the login dialog code makes the prompt insensitive
while while pam is processing the provided username, but the prompt
only makes itself sensitive again when it is hidden.

This commit makes it sensitive right before asking for a username again.

https://bugzilla.gnome.org/show_bug.cgi?id=740141

js/gdm/loginDialog.js

@@ -643,6 +643,8 @@ const LoginDialog = new Lang.Class({
                                                         realmManager.release();
                                                     }));
         this._updateCancelButton();
+
+        this._authPrompt.updateSensitivity(true);
         this._showPrompt();
     },