Bugfix: Mailing list mode disables PM emails (take 2)

Mailing List Mode disables normal emails, but mailing list mode does not email private_messages.

To avoid this, mailing list mode should only disable emails if they are not private_messages.

There are no tests for this commit as UserNotifications does not appear to be instantiated in the spec suite.
Merge pull request #2439 from peternlewis/mailing_list_mode_disables_private_message_emails2

Bugfix: Mailing list mode disables PM emails (take 2)