Resolve "Grading view improvements"

Closes #356 (closed) Closes #363 (closed) Closes #370 (closed) Closes #375 (closed) Closes #377 (closed) Closes #383 (closed)

Makes the following changes:

• Grading page now has only full data on the current submission and current problem. This forces proper, complete updates from the server when changing problem and / or submissions, ensuring a more up-to-date local state.
• Navigation and the decision of what submission to move to is now server side. This ensures that get next will actually produce an ungraded submission
• To allow for the progress bar, search box and problem selector, grading page has a metadata for all problems and submissions. For submissions, this metadata is id, student and for problems this is id, name. There is now no reason anymore to run call the costly updateAllSubmissions function in the Grading view anymore.
• The navigation is now a separate component, cleaning Grade.jsx a ton.
• Removed some code duplication (namely, generate problem data in one spot)
• Add docs for the Grade.jsx page