Fix an `ActionView::Template::Error: undefined method 'verification_key'
for nil` error in the welcome_user mailer when a user signs up without
an email address.
Caused by the fact that we now render mail templates regardless of
whether the user has an email address, and then skip sending the email
only after the mail template is rendered.
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.
