[ticket/12190] Add core event "core.modify_submit_post_data"
Add core event - core.modify_submit_post_data
to submit_post() function. This allows extensions to modify the
whole post data before the post submitting, or even provide
its own post submit procedure.

PHPBB3-12190
Merge pull request #2030 from rxu/ticket/12190

Add core event "core.modify_submit_post_data"