Create Python API for program_enrollments: Part III

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