new groups: fix student invites and leaving student led groups

fixes CNVS-13585
fixes CNVS-13588

test plan:

 - make sure new groups is enabled
 - as a student
 - create a new group and invite (student|students)
 - they should be the only students added to the group
 - the student should be able to leave and join that group

Change-Id: I8a62a6cb33b4774207d064d27a3b4471ad4dfb62
Reviewed-on: https://gerrit.instructure.com/36394
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: David Josse <david@instructure.com>
Product-Review: Drew Bowman <dbowman@instructure.com>