[ticket/13123] Add events to allow modifying post data

PHPBB3-13123
Merge pull request #3012 from nickvergessen/ticket/13123

[ticket/13123] Add events to allow modifying post data

* nickvergessen/ticket/13123:
  [ticket/13123] Add events to allow modifying post data
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/13123] Add events to allow modifying post data