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/helpers/users_helper.rb

@@ -1,4 +1,16 @@
 module UsersHelper
+  def unread_dmail_indicator(user)
+    "(#{user.unread_dmail_count})" if user.unread_dmail_count > 0
+  end
+
+  def has_unread_dmails?(user)
+    user.unread_dmail_count > 0 && (cookies[:hide_dmail_notice].to_i < latest_unread_dmail(user).id)
+  end
+
+  def latest_unread_dmail(user)
+    user.dmails.active.unread.first
+  end
+
   def email_sig(user)
     verifier = ActiveSupport::MessageVerifier.new(Danbooru.config.email_key, serializer: JSON, digest: "SHA256")
     verifier.generate(user.id.to_s)