Enhance enrolment record to include "expected start" and "expected end" dates

In "rolling enrollment" courses, there may be no single course start date. Because of this, the due dates for activities could be different for each student. MDL-61209 suggests that the due dates for activities be allowed to be set as deltas from the student enrollment date, but the date the student was enrolled in the course may not be their expected start date. It could be beneficial to modify the enrollment record so that it contains both an enrolled date and an "expected start" date. Activity deadlines would be set based on the "expected start" date, rather than the enrollment date. Similarly, the enrollment record could contain two ending dates, one switching the enrollee to read-only access, the other preventing any access to the course. Any "end" dates for enrollment could be calculated based on the "expected start" date. It is also possible that one might want to set due dates (and course enrollment expected start dates) relative to the enrollment end date.

This is key to supporting learning analytics for rolling-enrollment courses.