[ticket/12872] Add poster_id to viewtopic_modify_post_row

To allow extension authors to use the poster_id var.

PHPBB3-12872
[ticket/12872] Add poster_id to viewtopic_modify_post_row

Change "rc3" to "RC3".

PHPBB3-12872
Merge remote-tracking branch 'tmbackoff/ticket/12872' into develop-ascraeus