Frontend fails to change submission status if container stdout > max DB document size

If a student prints an amount of data between the maximum size allowed for a MongoDB document, and the maximum size allowed by the agent, then it is never stopped until the frontend fails to update the document in database because stdout output is stored in the document. The submission then remains in waiting state, preventing the student to submit.

There should be an additional check for that, probably directly in the agent as there should be no need for a >4MB stdout to be stored at the end, even for debugging.

Nothing prevents a too-long submission archive, but every component (Docker, backend, Mongo) can suffer from such a behaviour. Observed behaviour with GB produced archive was Docker socket timing out.