Add AASM gem

Both schedule_days and scheduled_questions are
going to act as state machines
Add aasm_state column to days and questions
ScheduleDays are now acting all state-machine-y
Make my schedule days fixture represent reality

Also pretty much use it.
Clean up these tests a bit.
Update state-machine-y-ness of ScheduleDay
Clean up schedule_day a bit...
Move ScheduleDay#first_potential into Participant

This puts it in the association where it belongs;
the "very first potential schedule day" means
nothing outside the context of a participant
Make ScheduledQuestions all state-machine-y

This will let us keep track of things that age out
for whatever reason.

Things will age out if we miss delivery due to an
outage or whatever.