Bugfix: Mailing list mode disables PM emails

Emailing a PM uses the user_posted event/format, but Mailing List Mode disables emails with this format.

We work around this by temporarily using "postedprivate", and then reverting back to "posted" efter the Mailing List Mode check.
Merge pull request #2420 from peternlewis/mailing_list_mode_disables_private_message_emails

Bugfix: Mailing list mode disables PM emails