dmails: allow marking dmails as unread.

* 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.

app/models/user.rb

@@ -35,6 +35,7 @@ class User < ApplicationRecord
   # - has_saved_searches
   # - opt_out_tracking
   # - enable_recommended_posts
+  # - has_mail
   BOOLEAN_ATTRIBUTES = %w(
     is_banned
     has_mail
@@ -798,14 +799,6 @@ class User < ApplicationRecord
     CurrentUser.as(self, &block)
   end
 
-  def dmail_count
-    if has_mail?
-      "(#{unread_dmail_count})"
-    else
-      ""
-    end
-  end
-
   def reportable_by?(user)
     ModerationReport.enabled? && user.is_builder? && id != user.id && !is_moderator?
   end