[Accessibility] Make group flash notifications accessible for JAWS

Fixes: CNVS-12797

JAWS requires a div to exist already for it to be
accessible. The flash_screenreader_holder existed,
so adding the partial there when it's needed will
make JAWS pick it up and read it immediately.

Test-Plan:
-Use old groups by disabling new group feature flag
  in account settings of admin account
-Create a group and make sure it's set for users to
  join automatically
-Login as user and join/leave group
-Verify that notification is accessible

Change-Id: I431ebcd74b7d01dea5041b801699b0ee72ead5ea
Reviewed-on: https://gerrit.instructure.com/36183
Reviewed-by: Drew Bowman <dbowman@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Jayce Higgins <jhiggins@instructure.com>