Add an API for submitting sketches to a class

[timothycpoon]

This would involve adding an API(s) to handler/class.go for submissions. This API would take in two PIDs (program IDs), a CID (class ID), and a UID (ID of the submitting user). For now, this API can call a mock function of the DB class which does nothing. The two PIDs correspond to the submission (student's program), and the assignment (teacher's program). The schema should match that of #97, but with the addition of the CID.

Your code should look similar to JoinClass up to line 192 or so, but with the correct inputs. The rest of the function can just be a comment or a dummy function call.