Hosts Can Take Attendance

Fixes #45.

Remaining:
- [ ] Hosts can see all tea times that have had attendance marked but not yet had a post-tea email sent to tea timers
- [ ] Hosts can mark an email to Tea Timers as having been sent
- [ ] Post-Tea emails are sent only after attendance is marked
- [ ] Hosts receive an email prompting them to take attendance ~30m after Tea Time is scheduled to end