Consider adding multiple enrollment-periods per offering

[jelmerderonde]

It is possible to have more than one enrollment-period for a single offering. For example: there could be a period for students studying at the same institution as the one providing the offering, and a different period for students from another institution.

One possible solution could be adding a new resource: /offering/{offeringId}/enrollment-periods

Enrollment periods should have information about "for which students" the enrollment period applies. Also of course start and end dates.