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
test that students are auto-assigned to cohorts in partition scheme
creation of a courseware preview menu to contain staff view toggle.
Extend preview to support cohorted courseware

TNL-651