Soft Delete for Courses

[Sumukh]

We should be able to do a "delete" of a course since we're letting anyone make a course.

I do not want to do a cascading delete of course data so that we can recover from an accidental deletion.

This involves adding a boolean for deleted on the model.

Maybe we'll change the query class for the Course model to just include the boolean in all queries. (See: https://github.com/miguelgrinberg/sqlalchemy-soft-delete/blob/master/app.py#L13 https://blog.miguelgrinberg.com/post/implementing-the-soft-delete-pattern-with-flask-and-sqlalchemy)