Hosts Can Take Attendance

Fixes #45.

Pre-Requisites for Merge:
- [x] Finalise and merge #248 (this branch is based off of that)

Remaining:
- [ ] Hosts receive an email prompting them to take attendance ~30m after Tea Time is scheduled to end
- [x] Hosts can see all tea times that have had attendance marked but not yet had a post-tea email sent to tea timers
- [x] Hosts can mark an email to Tea Timers as having been sent
- [x] Post-Tea emails are sent only after attendance is marked
  - [x] Fix #243 
- [x] Hosts can mark an attendee as a 'Potential Host'
- [ ] Styling, UX Review
It's actually attendances. Passed in in the TeaTimeFollowupNotifier. Will rename to make clearer.