generate UUID before adding group comments to submission

when adding group comments, we had been using a kludge
which attempted to add comments to a submission on
behalf of each student in a group, expecting the
exact same options hash every time. This fixes that
by moving generation of the group comment's unique ID
to the caller

Change-Id: I1920efffa2f5fc0150a4173abc1b8fa5490f2bd5
Reviewed-on: https://gerrit.instructure.com/29304
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Anthus Williams <awilliams@instructure.com>
Product-Review: Anthus Williams <awilliams@instructure.com>
QA-Review: Anthus Williams <awilliams@instructure.com>