Fixes for 0000032, 0000033 (prevent users from using mcom)

Disallow editing of posts/comments after someone else (a moderator) edits it.

Treat mcom as a bbcode error, and make sure that quoting a post with a mcom tag converts the tag into a quote tag. The existing code for handling advanced tags (e.g. for 'you' tags) was based on the original author of the post. To keep things simple, it was easier to just filter out mcode when posting rather than trying to calculate it based on whoever edited the post.