[ticket/12289] Add viewtopic template events for posts specific custom notices

These events allow extensions to add own posts specific notices-alike info.

PHPBB3-12289
Merge remote-tracking branch 'rxu/ticket/12289' into develop-ascraeus

* rxu/ticket/12289:
  [ticket/12289] Add viewtopic template events for posts specific custom notices
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12289] Add viewtopic template events for posts specific custom notices