improve student group signup experience

fixes CNVS-11832
fixes CNVS-11836
fixes CNVS-13327

test plan

  - enable feature student_groups_next for course
  - regression test groups page student view

  - regression test instructor groups page

  - as an instructor have multiple groups in a group set
  - visit one of the groups
  - the instructor can switch between the groups in the group set

  - as a student you should be able to create a group

Change-Id: Iacf1eaf4467b57f307d45245b53dfc93f14154ff
Reviewed-on: https://gerrit.instructure.com/34454
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ethan Vizitei <evizitei@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Drew Bowman <dbowman@instructure.com>