Enable maintenance-mode exercises that aren’t visible in ToC

[jsorva]

AFAIK, there’s currently no way to put an exercise in maintenance mode while simultaneously hiding it from the ToC. However, that’s precisely what I’d normally like to do when I put an exercise in maintenance mode, and I know of other teachers who’d like to do the same.

(Context: My exercises aren’t normally listed in the ToC — but they very visibly appear there if they’re put into maintenance mode, which is counterproductive.)

Suggestion: either hide all maintenance-mode exercises from the ToC or add another option to the list of exercise statuses.

[jsorva]

A closely related old issue: #438