API Endpoint: Direct/Proxy Submission

[eriq-augustine]

Create an emdpoint that allows a grader to directly submit a student's submission.
Override rejection and late times.
We should probably also be allowed to just set the submission time manually (but note that is was a direct submission).
Or maybe we can just tell the autograder to never mark this as late.

The key use case here is when a student's submission situation is irregular, but the instructor still wants to accept it.
For example, a student joining a class late.

It would be really cool if we could either upload files, or just provide a submission id for one of the student's already existing submissions.

[lkuper]

+1 for this feature!