[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.

PHPBB3-12403
Merge remote-tracking branch 'github-rxu/ticket/12403' into develop-ascraeus

* github-rxu/ticket/12403:
  [ticket/12403] Add template events to acp_users_prefs.html