Reuse of the variable paid_modes was causing the code to follow an
incorrect path. Changed the filtering version of the variable to
paid_modes_only. Had to keep both variables temporarily until I can fix
plugin code that uses this API.
Checking if a user was enrolled in a program type was using the `name`
field which is subject to be translated. This change allows for us to check by
the type's slug which will be constant. This also includes the addition
of api.py files for the course_modes, catalog, and programs apps.
