Fix an exception when the user's unread_dmail_count is nonzero but they
don't have any unread dmails. This normally shouldn't happen unless the
unread_dmail_count is inconsistent.
* Add ability to mark dmails as unread.
* Fix users.unread_dmail_count to not count deleted dmails.
* Fix show action so that API calls don't mark dmails as read.
* Don't show the unread dmail notice on the /dmails page itself.
* Stop using users.has_mail flag.
