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
creation of a courseware preview menu to contain staff view toggle.
Extend preview to support cohorted courseware

TNL-651
Masquarading must be set up on xblock handler callbacks.

Change masquerade test so it more clearly tests the behavior of masquerading as a student.
Method got lost in merge.