don't load audience enrollments for conversations index

fixes CNVS-15886

test plan
- ensure that replying to a contextless conversation (from old
 conversations) still works. you'll have to make one using the
 console

Change-Id: Icdde36a0a2ed278523a293c960982bf317f5df12
Reviewed-on: https://gerrit.instructure.com/42048
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>