conversations: fix recipient pagination caching

fixes CNVS-11199

test plan:
  * open conversations as a user who can message
      more than twenty other users
  * open compose dialog
  * open recipients browser
  * open context with many users and scroll down
  * verify that all users load
  * close and re-open recipients browser
  * verify that all users are still present

Change-Id: Ibeefddc92f49cc52195336eee50d7213e8717d12
Reviewed-on: https://gerrit.instructure.com/36381
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Braden Anderson <braden@instructure.com>