Add digest e-mails

app/controllers/api_controller.rb

app/controllers/settings/preferences_controller.rb

app/lib/formatter.rb

app/mailers/notification_mailer.rb

app/models/setting.rb

app/models/user.rb

app/views/layouts/mailer.text.erb

app/views/notification_mailer/_status.text.erb

app/views/notification_mailer/digest.text.erb

app/views/notification_mailer/favourite.text.erb

app/views/notification_mailer/follow.text.erb

app/views/notification_mailer/follow_request.text.erb

app/views/notification_mailer/mention.text.erb

app/views/notification_mailer/reblog.text.erb

app/views/settings/preferences/show.html.haml

app/workers/digest_mailer_worker.rb

config/application.rb

config/environments/production.rb

config/locales/en.yml

config/locales/simple_form.en.yml