Add base support for cohorted group configurations

TNL-649
Add support for user partitioning based on cohort.

JIRA: TNL-710

IMPORTANT: this commit converts the course_groups
package to using migrations.  When deploying to an
existing openedx instance, migration 0001 may fail
with an error indicating that the CourseUserGroup
table already exists.  If this happens, running
the 0001 migration first, with the --fake option,
is recommended.  After performing this step,
remaining migrations should work as expected.
Add group_access field to all xblocks

TNL-670
avoid errors when forward-migrating course_groups after south conversion

JIRA: TNL-937
Merge pull request #6160 from edx/jsa/tnl-937

avoid errors when forward-migrating course_groups after south conversion
Document the Open edX package.
Merge pull request #5942 from edx/cohorted-courseware

Cohorted courseware