[ticket/12828] Add ucp_prefs.php common core event

Add common core event at the bottom of the includes/ucp/ucp_prefs.php
to allow additional actions (like template variable assignments etc.)
before the page load.

PHPBB3-12828
[ticket/12853] Change navbar ACP from skip- to last-responsive

PHPBB3-12853
[ticket/12845] Remove invalid ARIA role for breadcrumbs

PHPBB3-12845
[ticket/12837] Detect "viewing contact admin page" on viewonline

PHPBB3-12837
[ticket/12837] Use else if instead of elseif

PHPBB3-12837
Merge pull request #2709 from rxu/ticket/12828

[ticket/12828] Add ucp_prefs.php common core event

* rxu/ticket/12828:
  [ticket/12828] Add ucp_prefs.php common core event
[ticket/12859] Add template events ucp_pm_viewmessage_post_buttons

PHPBB3-12859
Merge pull request #2743 from PayBas/ticket/12859

[ticket/12859] Add template events ucp_pm_viewmessage_post_buttons

* PayBas/ticket/12859:
  [ticket/12859] Add template events ucp_pm_viewmessage_post_buttons
Merge pull request #2741 from PayBas/ticket/12837

[ticket/12837] Detect "viewing contact admin page" on viewonline

* PayBas/ticket/12837:
  [ticket/12837] Use else if instead of elseif
  [ticket/12837] Detect "viewing contact admin page" on viewonline
Merge pull request #2740 from PayBas/ticket/12845

[ticket/12845] Fix invalid ARIA role for breadcrumbs

* PayBas/ticket/12845:
  [ticket/12845] Remove invalid ARIA role for breadcrumbs
Merge pull request #2739 from PayBas/ticket/12853

[WIP][ticket/12853] Change navbar ACP from skip- to last-responsive

* PayBas/ticket/12853:
  [ticket/12853] Change navbar ACP from skip- to last-responsive