* make a new pref for activity / daily update email that is separate from the
  messages preference
* add experimental UI in Account Settings
* support control over select Sendgrid suppression groups from Account
  Settings
* refactor EmailSuppression#destroy_remote to use SendgridService

…re needed

* support new unsubscribe groups for activity, message, and onboarding emails
* continue support for the transactional group for now, i.e. we will not
  deliver a daily update email if the user is suppressed in the transactional
  group, even if they are not suppressed in the activity group
* send cohort experiment emails with the onboarding group, which means users
  can unsubscribe from them
* fix bug in which domain_is_not_supported error for users was not being
  correctly reported or translated

* Set up a webhook endpoint for use with Sendgrid, secured by OAuth
* Process unsubscribe and resubscribe events to create/destroy
  EmailSuppression records for local use, and keep
  prefers_message_email_notification setting in sync until we can remove it
* Add temporary callback in User to try to keep suppressions in sync with the
  prefers_message_email_notification pref
* Require email attribute for all EmailSuppression records, and attempt to
  populate it from the associated user on create
* Remove new prefers_activity_email_notification setting, which is redundant
  with the EmailSuppression record (so is the messages one, but that's
  already in prod)
* Refactor User#suppressed_in_group? for clarity (too much multiple negation)
  and to get it to work the way it seems like it was intended to work
  (unnecessarily restricted to a set of groups)
* Add admin-only section to Account Settings for access to these finer
  controls