Rewritten doc, update_database returns submission and exam, pregrading now checks for specific submission

Changed commented out code back, changed the way errors are printed, added temporary fix for box check, feedback description should be the label of the mc_option if it is present

Merge branch 'combinatory-fixes' of https://gitlab.kwant-project.org/works-on-my-machine/zesje into feature/pre-grading

Fix unable to save feedback with score 0

Closes #199

See merge request zesje/zesje!151

Set scan processing status while waiting for a worker

See merge request zesje/zesje!149

Add scaling arguments to celery for yarn dev

See merge request zesje/zesje!148