Remove tutor column from user model.

This removes the existing boolean tutor column, as it is now
unnecessary and clashes with the Tutor model relation.
User creates client instance on save.

*All* users have the client role.  Tutor and admin roles default to
nil.
User#tutor? helper.

Predicate to check if the user is a tutor, saves having to use
`user.tutor.nil?` everywhere.
Do not build extra Client in client spec.
Add subject model.

Subjects taught by tutors.