Problem exclusion field

[Xyene]

We should add a field mapping a problem to a list of "exclusion" problems. This would solve issues where we have both standard and signature-graded IOI problems, as well as *hard variants of problems giving "double" the points for solving the hard version.

When calculating user points, the max point value between problem exclusions should be taken. The exclusions should be displayed on the problem page, probably below "Allowed languages".

[quantum5]

Remember to use a symmetrical ManyToManyField on 'self'.

[Ninjaclasher]

Some things to remember:

• handle problem exclusions on user PP breakdown page
• recompute user points when problem exclusion changes from admin
• handle problem exclusions for solved problem count (?)
• add problem exclusions to API