* Minimum possible changes were made to merge CapaModule & CapaDescriptor into
  one ProblemBlock class.
* There are no known changes in behavior.
* CapaModule and CapaDescriptor inherited from a number of classes which inherit
  from XModule or XModuleDescriptor but did not depend on them. For all these
  classes the methods were moved to mixins which did not inherit from either and
  then these mixins were added to ProblemBlock in the order which maintains MRO.