[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
[ticket/12180] Add "Each file MUST end with exactly one newline character" rule

PHPBB3-12180
Merge remote-tracking branch 'bantu/ticket/12180' into develop

* bantu/ticket/12180:
  [ticket/12180] Add "Each file MUST end with exactly one newline character" rule
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