[ticket/12584] Add event core.mcp_warn_user_before/after

Event for when warning a user from MCP.

PHPBB3-12584
[ticket/12584] Redirect only on non-empty $message

PHPBB3-12584
Merge pull request #2488 from n-aleha/ticket/12584

[ticket/12584] Add event core.mcp_warn_user_before/after

* n-aleha/ticket/12584:
  [ticket/12584] Redirect only on non-empty $message
  [ticket/12584] Add event core.mcp_warn_user_before/after