FEATURE: More granular mailing list mode (#4068)

* Rearrange frontend to account for mailing list mode

* Allow update of user preference for mailing list frequency

* Add mailing list frequency estimate

* Simplify frequency estimate; disable activity summary for mailing list mode

* Remove combined updates

* Add specs for enqueue mailing list mode job

* Write mailing list method for mailer

* Fix linting error

* Account for stale topics

* Add translations for default mailing list setting

* One query for mailing list topics

* Fix failing spec

* WIP

* Flesh out html template

* First pass at text-based mailing list summary

* Add user avatar

* Properly format posts for mailing list

* Move make_all_links_absolute into Email::Styles

* Apply first_seen_at to user

* Send mailing list email summary hourly based on first_seen_at

* Branch and test cleanup

* Use existing mailing list mode estimate

* Fix failing specs

app/serializers/user_option_serializer.rb

@@ -2,6 +2,7 @@ class UserOptionSerializer < ApplicationSerializer
   attributes :user_id,
              :email_always,
              :mailing_list_mode,
+             :mailing_list_mode_frequency,
              :email_digests,
              :email_private_messages,
              :email_direct,