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
Merge pull request #6274 from edx/jsa/add-cohort-scheme-coverage

test that students are auto-assigned to cohorts in partition scheme
Merge pull request #6175 from edx/jsa/xblock-group-access-2

block.get_parent() and group access enforcement
Studio: adding UI and styling for component-level access modal
fixup! addressing UI review/hand off feedback:

* removed summary header UI element
* change "access" to "visibility"
* change second radio element to "Specific Content Groups"
* replaced key icon with eye icon
* synced up specific visibility-related styling (sidebar + active icon state)
* synced up all developer (NOTE: @andyarmstrong...) note mentions
fixup! addressing FED/PR feedback
Studio support for cohorted courseware

TNL-652
Rewrite to use xblock views
fixup! revising modal markup/styling to note xblock rendering conventions
fixup! copy/messaging changes from wise docs folks
Update template
Update text in publishing component and state of "eye" when visibility is limited to specific content groups.
Read-only version of modal
Add python unit tests for AuthoringMixin
fixup! Add python unit tests for AuthoringMixin
fixup! Add python unit tests for AuthoringMixin
Implemented save
Add jasmine test for creation of visibility modal.