[ticket/12403] Add template events to acp_users_prefs.html

Since it is possible for extensions to add new user preferences to UCP,
it should be also possible to control it via ACP too
just like for build-in user preferences.
Thus, some prepend/append template events are added.

<a href="https://tracker.phpbb.com/browse/PHPBB3-12403">PHPBB3-12403</a>.