Buttons to navigate between problems in contest

[Xyene]

It's possible to display something of the form <| [Current contest] |> on the header of a problem, such that navigating between contest problems is easier. This can trivially be extended to have the same available out of contest for problems that belong to contests.

Will likely need a check in to prevent a problem being added to two contests, as this display would not work.

[quantum5]

The whole system is designed so that you CAN put a problem into multiple contests. Why I don't know, but it's designed like that.

[FatalEagle]

Why would you want to put a problem into multiple contests? Division 1/ Division 2.

[Xyene]

Whether we choose to add this or not, we should decide and implement the decision before the October DMOPC.

[Xyene]

This was requested again, and would play nice with the metadata we'd gain in #478.

[quantum5]

The problem that a problem can be in multiple problems contests remains. This is especially annoying since we do have contests that pull in random CCC and COCI problems, and that would make it impossible to have unique problem ordering.