Enrollment data handling fixes

[johnbaldwin]

This PR has fixes for the new enrollment data processing

Fix backfill_enrollment_data_for_course task.Commit e3faba1

• Problem was that update_enrollment_data_for_course returns a list of tuples. Each tuple is (object, created), where object is the EnrollmentData record and created tells if the record was created
• Added logging to the backfill task
• Updated tests

Fix class for Python27 Ginkgo. Commit bf83408

• one liner makes test class inherit from object, since we still need to support py27.

Adds enrollments filter method to Figures Course class. Commit bd03842

• New method figures.course.Course.enrollments_with_student_modules
• This method finds CourseEnrollment records for a course for enrollments that have StudentModule records in the course. This functionality helps simplify finding out of date EnrollmentData records.

Add test case to tests/test_course.py . Commit: 2566818

• Was missing the test case when there were no student modules in the course. We want to make sure our "empty set" handling is correct

Add stale course enrollment handling to pipeline. Commit ded1349

• What's included here is a generator to yield CourseEnrollment records that need to have their EnrollmentData records updated
• If we install Figures on a site that's already been running or we miss a day or more in processing, then the daily processing will not pick up on all "stale enrollments", enrollments that Figures EnrollmentData record updated.
• This is because Figures daily metrics jobs only look at "yesterday" in order to work around the limitations of StudentModule.
• The real logic is in figures.enrollment.is_enrollment_data_out_of_date (Which is still pretty simple)

Fixed figures.tasks.backfill_enrollment_data_for_course. Commit 3511f2b

• Now it looks for stale course enrollments and for each stale course enrollment found, updates that enrollment's metrics. Prior to this commit, it was just calling the normal daily enrolment data update function.
• Updated unit tests

Update backfill enrollment management command. Commit cdeb6a2

• Add ability to load course ids from a file
• Add test coverage

One last thing to check: The student module GTE check in figures.enrollment.student_modules_for_enrollment_after_date

[johnbaldwin]

Thanks @OmarIthawi !