[MICROBA-1508]
- Adds the InstructorTaskSchedule model
- Adds the HistoricalInstructorTaskSchedule model
- Adds utility function used to create a InstructorTaskSchedule instance
- Adds a public `create_course_email` Python API function to the bulk_email app that can be imported and used in external apps
- Adds a new `test_api_helper.py` test file (with tests for the new `schedule_task` function) to the instructor_task app
- Adds a new `test_api.py` test file (with tests for the new `create_course_email` function to the bulk_email app
Django 2.0 will make this field required for `ForeignKey` and `OneToOneFields`.
In previous versions the option defaulted to `models.CASCADE` when not
specified. This change should make the deprecation warnings in the current
Django version go away.
The migrations where also modified, but the changes should not cause a change in
the database schema since `models.CASCADE` was already the old default.
