[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