clean out existing non-observer enrollments with an
associated_user_id, and add a validation check preventing
these from being created in the future
refs #10652
Change-Id: Id1887789786fd09f2df64bc6432c81bbffb7e136
Reviewed-on: https://gerrit.instructure.com/13786
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Jeremy Stanley