Get users from mailing lists

- A. **import all emails**
- unsubscribe button
- convert to StartSCH button
- B. **add an easy sign-up flow**
- [ ] #23
- similar to `/preferences`, show group name, email/push checkbox and sign-in button
- redirect to authsch, then subscribe, ask for notification permission (requires user click), then end at `/preferences` where show a success banner

Start with issue #23 and the existing `/preferences` flow, then map the mailing-list import and the sign-up redirect through authsch. Done means emails can be imported with unsubscribe and StartSCH actions, while new users can choose email or push notifications, grant notification permission, and finish at `/preferences` with a success banner.

Written by the indexing model from the issue text.