filter conversations with no active messages

fixes CNVS-9325

test plan:
  * as user A in new conversations, send multiple messages
    to user B;
  * as user B in old conversations, delete all of the
    messages inside one of the new threads;
  * reload the page and verify that you can click on each
    of the conversation threads in the left pane and that
    they load as expected.

Change-Id: I531bb4e73c19d0c927dd5a033a46ced76aded512
Reviewed-on: https://gerrit.instructure.com/26059
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>