Create Python API for program_enrollments: Part IV

This commit completes the program_enrollments LMS app
Python API for the time being. It does the following:
* Add bulk-lookup of users by external key in api/reading.py
* Add bulk-writing of program enrollments in api/writing.py
* Move grade-reading to api/grades.py
* Refactor api/linking.py to use api/writing.py
* Refactor signals.py to use api/linking.py
* Update rest_api/v1/views.py to utilize all these changes
* Update linking management command and support tool to use API
* Remove outdated tests from test_models.py
* Misc. cleanup

EDUCATOR-4321