cache 'everyone' search result in new conversations

fixes CNVS-12865

test plan
- as a teacher in new conversations, compose a new message
- select 'all in course' from the people picker drop down
- ensure that 'all in course' can be selected
- ensure that the message can be sent and is delivered to all
 users in the course

Change-Id: I6f2e0ad30ca2a09e52b4b5b764f3c21ecc0628f0
Reviewed-on: https://gerrit.instructure.com/34291
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>