emails: fix deliver_later mails not being sent.

By default, mails sent by deliver_later are added to the `mailers` job
queue. These mails weren't being sent because workers were only
configured to process the `default` queue.

config/application.rb

@@ -36,6 +36,8 @@ module Danbooru
     raise "Danbooru.config.secret_key_base not configured" if Danbooru.config.secret_key_base.blank?
     config.secret_key_base = Danbooru.config.secret_key_base
 
+    config.action_mailer.deliver_later_queue_name = "default"
+
     if Danbooru.config.mail_delivery_method.to_sym == :smtp
       config.action_mailer.delivery_method = :smtp
       config.action_mailer.smtp_settings = Danbooru.config.mail_settings