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

db/migrate/20160309073132_add_mailing_list_mode_frequency.rb

@@ -0,0 +1,5 @@
+class AddMailingListModeFrequency < ActiveRecord::Migration
+  def change
+    add_column :user_options, :mailing_list_mode_frequency, :integer, default: 0, null: false
+  end
+end

db/migrate/20160326001747_add_user_first_visit.rb

@@ -0,0 +1,5 @@
+class AddUserFirstVisit < ActiveRecord::Migration
+  def change
+    add_column :users, :first_seen_at, :datetime
+  end
+end