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.
