Factor the core Capa problem logic into a mixin.
This mixin can be used by either the current XModule, or by an XBlock. Leave
the XModule-specific code in capa_module and move the shared code to capa_base.
Plan to move caba_base into the capa package in the future.
Merge pull request #2124 from edx/jorr/feature/factor-shared-capa-features-into-mixin

Factor the core Capa problem logic into a mixin.