Fix page load error on Account level group discussions
Fix page load error on Account level group discussions

Group.all_real_students will return all account users when a group's
context is an Account

Fixes CNVS-17623

Test plan

Regression Test
  - Account Level Discussions
  - Group Level Discussions

Change-Id: I0db79ab8285f264d46bce82ddd2a020d894befcd
Reviewed-on: https://gerrit.instructure.com/46974
Reviewed-by: Joel Hough <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Steven Shepherd <e794458448443812fb2685675aab3bbd1a3891bc@instructure.com>
Product-Review: Brad Horrocks <7b5c5d1f2eba6c540b8e53f17d87a9bd7dbf604b@instructure.com>

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