Add teacher endpoint for getting StudentWork from PeerGroup

[hirokiterashima]

Create a new endpoint to allow teachers to retrieve student work for a specific PeerGroup. They have access to runId, workgroupId, nodeId and componentId, which should point to a PeerGroup object.

Secured: Teacher role
Method: GET
URL: /api/peer-group/{runId}/{workgroupId}/{nodeId}/{componentId}/student-work
Return a list of StudentWork objects from members in the PeerGroup

List<StudentWork> getPeerGroupWork(Long runId, Long workgroupId, String nodeId, String componentId)

[hirokiterashima]

Closed by #50