student leader group management

fixes CNVS-13404

test plan
- as a teacher, create a group with a student group leader
- as the group leader, view the groups page
- ensure that the group has a badge indicating that you are the
 group leader
- ensure a 'Manage' link appears next to the 'Visit' link
- using the 'Manage' link, edit the group name and add/remove users
- ensure that the group is edited
- set a group member limit
- try to add more members than the limit
- ensure that a helpful error is shown when you try to submit
- ensure that trying to submit a group with no name shows a helpful
 error

Change-Id: I5fdf242f21a7f353aaceb20fb75eed2833b197b9
Reviewed-on: https://gerrit.instructure.com/45259
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>