* 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.