Split AcidBlock into leaf and parent variants

I've refactored AcidBlock into two xblocks:
* AcidBlock - a leaf xblock that tests itself
* AcidParentBlock - a parent xblock that tests itself and its children

The intention is to update our Bok Choy tests to verify Studio's behavior with both types of xblock. I'll submit the new tests as a separate PR.

@cpennington @nedbat Please review.