loginDialog: support disable-user-list key

In some deployments showing a user list at the login
screen is undesirable.

GDM's fallback login screen has a configuration key:

org.gnome.login-screen disable-user-list false

that causes the user-list to get hidden.

This commit adds similar functionality to the normal,
shell-based login screen.

Based on a series of patches by Marius Rieder.

https://bugzilla.gnome.org/show_bug.cgi?id=660660

js/gdm/util.js

@@ -23,6 +23,7 @@ const BANNER_MESSAGE_TEXT_KEY = 'banner-message-text';
 const ALLOWED_FAILURES_KEY = 'allowed-failures';
 
 const LOGO_KEY = 'logo';
+const DISABLE_USER_LIST_KEY = 'disable-user-list';
 
 function fadeInActor(actor) {
     if (actor.opacity == 255 && actor.visible)