Previously, ProblemBlock's implementation was split between
CapaMixin/CapaFields (in capa_base.py) and ProblemBlock
(in capa_module.py), the former being the base classes of the
latter. The split existed for a historical reason:
as a former XModule, ProblemBlock was once split
between CapaDescriptor (author-facing) and CapaModule
(learner-facing). Since ProblemBlock has been converted
to being a pure XBlock, the division between the base
classes and the block class are no longer necessary
nor semantically helpful.
docs: Flesh out ProblemBlock's docstring a bit.
b0d41f6ebb