allow users to manage group membership limits

fixes CNVS-11830
fixes CNVS-11831

test plan

 - add a course group
 - set the max members in the group creation dialog
 - verify that the limit is enforced

 - enforced it a course group
 - change the max members in the group edit dialog
 - verify that the new limit is enforced

 - edit the group set
 - change the max members limit for the group set
 - verify that the limit is changed for all of the groups in the set

Change-Id: I6f112d29d6c5ca662da1598e29db2f3081f30148
Reviewed-on: https://gerrit.instructure.com/33722
Reviewed-by: Joel Hough <joel@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Drew Bowman <dbowman@instructure.com>