0000033: Disallow editing after a mod edits a post

Funnel all checks for comment editing through the newly-added
functions can_edit_comment and validate_edit_comment in the
newly-added classes/class_comment.php.

Regular users will not be able to edit a comment if it has been
edited by another user (generally only a mod can edit comments)
0000032: Don't allow users to use [mcom] tag in forums/comments

Treat [mcom] tags as bbcode errors for non-mod users.

When quoting a post with [mcom], convert to [quote] tag before
loading in bbcode editor.
Merge pull request #21 from lazyblum/work

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