Implement blackout appointments.

A blackout appointment marks hours when a tutor is not working.

This *probably* doesn't belong in here, but this is the easiest way to
do it.  Still need view / controller code to interact with this.
Add seed data.

Rough user/subject/tutor seeds, should save time compared to setting
things up manually after each migration.
All tutors tutor 'Other' by default.

'Other' is a catch-all for appointments which may not fall exactly
within a tutor's official subjects

Tight coupling in tests is beginning to cause problems.