Remove the restrictions on the challenge automatically

[nickkolok]

See also #281

When no problems are available for challenge, ееру куstictions are removed one-by-one. As for now, the standard list (on the Round detail page) does not reflect that; however, the cancelled restrictions should be striked out.

The tactics builder raises the Problems that became available first, to the top of the list of forbidden problems. However, that's not enough: they should be marked as available, and the reason for which they had been forbidden should be also striked out.