Fix possible IntegrityError when deleting task that is part of a contest

[prandla]

#1065 noticed this bug and fixed it for the "remove task for contest" case. However, the same logic was in two other places that could still cause the error.

For the bug to happen, you need the ID order of the shifted tasks to not match the order they have in the contest. In this case, psycopg apparently attempts to update the tasks in order of ID, which causes an integrity error after the first update, because the updates aren't batched properly.

[prandla]

I noticed #469 which seems to also be related to the requirement that num is always in the range 0 to n-1. That issue is probably fixed by now, but evidently keeping nums updated properly isn't trivial. I think it might be simpler to drop this requirement altogether; "move to end" can set num = max(nums) + 1, "move to front" sets num = min(nums) - 1, move up/down already just swap the nums without needing them to be consecutive. This way, moving or deleting things doesn't require fiddling with the numbers on other tasks at all.