Allow verification of submissions on (contest)problem level

[vmcj]

This allows for checking of the results and already allowing for some problems when enough submissions have come in.

As suggested by @mkfuron in https://domjudge-org.slack.com/archives/CHHURFUM7/p1779496255283309

That discussion also had other options by @mpsijm which I think are different ways of looking at the problem so I suspect we should support both (or neither?).

Also added some minor fixes:

[cubercsl]

BTW, this feature was brought up in #645 before, so I think someone might switch the related settings during the contest. So I think we need to test this kind of scenario.

[mpsijm]

Thanks for pulling me in here! It looks like there are two separate concerns:

• This PR currently appears to move the global setting "Verification required – Is manual verification of judgings by jury required before publication?" to the problem level. This setting keeps submissions as "pending" as long as they are not manually verified.
• During BAPC/NWERC, we always have this setting off, but still manually verify submissions after their result has already become visible (so, moving this setting to be per-problem will probably not change anything for us). However, this becomes more difficult to manage once the easier problems have been solved many times. My suggestions in https://domjudge-org.slack.com/archives/CHHURFUM7/p1779306901059579 (the message before Marc's) were ideas for improvements in this scenario 🙂

[vmcj]

* This PR currently appears to move the global setting "Verification required – Is manual verification of judgings by jury required before publication?" to the problem level. This setting keeps submissions as "pending" as long as they are not manually verified.

Yes, I think Marc handles this different, so I first wanted to implement his suggestion and address your suggestions in a future PR as it requires a bit more work to do it cleanly/performant.

And it shouldn't move it, it should add an additional option to do it either globally or for specific problems.