The first new user is automatically made into an instructor

This resolves issue #62.

Note that I had to change the few tests that do not use signin_as so that they would either register a dummy instructor before registering any students, or register the instructor first if an instructor was involved in that test. I get the feeling that maybe there could be a nicer way of doing this, though if we made it so that an instructor was automatically registered before *every* test, we wouldn't be able to actually test the function where the first user is made into an instructor.
I would be inclined to have this be a `before_create` so that I could use `User.all.empty?` and just set instructor via the usual setter.