This is the third in a series of commits to create
a Python API for the LMS program_enrollments app.
It does the following:
* Creates api/ folder.
* Moves link_program_enrollments.py to api/linking.py
* Creates api/reading.py for enrollment-fetching
functions.
* Updates rest of app to use api/reading.py when
it was going directly through the models before.
* Other misc. cleanup (isorting, unicode_literals,
line breaks, etc).
Still to do:
* Create api/writing.py and update app to use it instead
of going directly through models.
* Create api/reset.py and api/expire.py, which the management
commands call out to.
EDUCATOR-4321
ProgramCourseEnrollments were already unique on
(program_enrollment, course_enrollment) by nature of the
OneToOneField on course_enrollment. However, this only
affects realized enrollments. For waiting enrollments, we
need to add a uniqueness constraint on
(program_enrollment, course_key).
