Remove keep trailing slashes from Angular config.

Angular likes to strip trailing slashes from URLs. This didn't play well
when the API urls were all configured with trailing slashes. Now that
the Flask-Restful refactor is in place, API urls are configured without
trailing slashes, so this option is not needed anymore.
Users avatar property now a hybrid property again

SQLAlchemy's hybrid_property option didn't play well with
Flask-Restless. Now that we're using Flask-Restful, we can use
hybrid_property again instead of having to call Users.avatar().
Convert Courses API to use Flask-Restful.

This is the last API to be converted. The Flask-Restless based helper
code has also been removed.

Flask-Restful requires you to specify the fields that are transmitted to
the user. This now lives in the dataformat module. Would've named it the
fields module except then we'd have a confusion with Flask-Restful's
fields module.