[ticket/12132] Add viewtopic_print_head_append event to subsilver2

PHPBB3-12132
[ticket/12132] Add the event to events.md doc

PHPBB3-12132
[ticket/12134] Add ucp_pm_viewmessage_print_head_append event to subsilver2

PHPBB3-12134
Merge remote-tracking branch 'vinny/ticket/12132' into develop

* vinny/ticket/12132:
  [ticket/12132] Add the event to events.md doc
  [ticket/12132] Add viewtopic_print_head_append event to subsilver2
Merge remote-tracking branch 'vinny/ticket/12134' into develop

* vinny/ticket/12134:
  [ticket/12134] Add ucp_pm_viewmessage_print_head_append event to subsilver2