make block.get_parent() work.

Co-Authored-By: Christina Roberts <christina@edx.org>
Co-Authored-By: Daniel Friedman <dfriedman@edx.org>
Co-Authored-By: Don Mitchell <dmitchell@edx.org>
support group_access.  JIRA: TNL-649
Merge pull request #6175 from edx/jsa/xblock-group-access-2

block.get_parent() and group access enforcement