Add Student to identity of StudentAssessment (for v2.x)

This is a clone of DATASTD-1231, to ensure this change is captured for the 2.x line. Original description quoted below

Per DATASTD-1214, the identity of StudentAssessment was changed to StudentAssessmentIdentifier and AssessmentIdentifier.

Arguably, Student is also a useful and strongly identifying property (a change to student would result in a new entity). This could also provide continuity to implementations that make use of the fact that Student is a key entity (this has been shown already in the case of the Ed-Fi API implementation).

For API clients, there are virtually no changes. Student is already a required property and so referential integrity is already enforced. This _could _be impactful if a client is trying to re-assign a result to another student, but such cases would be very rare and could be handled efficiently via delete/creates.