support group_access.  JIRA: TNL-649
staff are exempt from group access checks (squash me)
remove unused function
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.
Bok choy test for "previewing as" selector in LMS.