Sessions endpoint needs to return assessment records in addition to attendance

**Dependency**
Coach App

The app will create assessment records which link a student to a session to an assessment value/response.
The coach needs to quickly see for which students there are records so they can complete the data collection for the entire roster, much like how the app displays attendance status.

Ideally the endpoint returns a list based on the session roster. We should follow the same pattern used in the creation and query of attendance records, combining student and session ID's into a unique. This way potential for multiple assessment records per session is protected (presumably using dynamic third value from the DB).