Revert "Change to ActiveRecordStore sessions."

This reverts commit 1d1e8035b66442a89bfb6efa84d527c08bb68d98.

Spoke to Wes at Trike. He gave me reasons to stick to cookie store (it's
what Trike always uses, principle of least surprise), and ways to not
need active record store (stick stuff in the model, obvious in
hindsight).
Add model for credence game.

There's a db table, but currently I don't know enough about rails to
know whether anything gets put in it.

For now we're still storing the user's game in the session.

A game has a score, and questions are now random.