There are 3 main changes in this commit:
* CohortFactory now sets up memberships properly, so consuming tests do not
need to explicitly touch CourseUserGroup.users to add() users.
* test_get_cohort_sql_queries has been updated to 3 and 9 queries when using
and not using the cache, respectively. This is needed due to each operation
needing an extra queery to get the CourseUserGroup from the CohortMembership.
* Adding remove_user_from_cohort(), the counterpart to add_user_to_cohort().
This is also to keep tests from touching the users field directly, and keep
CohortMembership data in sync.

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.

TNL-649

Move modulestore config for tests to an importable location
Disable pylnt warning for lms imports in common tests
Refactor all testcases that loaded all xml courses
TE-610
TE-489

Add explicit course key to enrollment for non-existent course in test case

Previously, the user endpoint would would return 404 for any request
involving a user having an enrollment for a course that no longer
existed.

This new endpoint is designed specifically to fill the needs of the
notifier and should not be used by other clients.