Acceptance flow

This is the basic version of it with database additions, control buttons on admin (with the view user page cleaned up just a little), and the user-side confirm thing. Some text is placeholder like the doodle link.

As of first push, this doesn't automatically send emails. I want to get that working in the morning, but if it isn't done in time, we can still manually send them with mail merge or whatever, so this PR - once the placeholders are swapped out - can represent a MVP.
This is really "availability_requested" or "availability_confirmation_requester", right?