[ticket/12583] Add event core.mcp_warn_post_before/after

Event for when warning a user for a post.

PHPBB3-12583
[ticket/12583] Redirect only on non-empty $message

PHPBB3-12583
Merge pull request #2487 from n-aleha/ticket/12583

[ticket/12583] Add event core.mcp_warn_post_before/after

* n-aleha/ticket/12583:
  [ticket/12583] Redirect only on non-empty $message
  [ticket/12583] Add event core.mcp_warn_post_before/after