conversations: support messages to account-level groups

fixes CNVS-10776

test plan:
  * log in as a user in an account-level group
  * open conversations
  * compose a message
  * verify that you can address the message to the group

Change-Id: Icb3d840e1d17cabb94394cc564f6d3c5b5f606e0
Reviewed-on: https://gerrit.instructure.com/35309
QA-Review: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ethan Vizitei <evizitei@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>