fix new conversation autocomplete

fixes CNVS-10686

test plan:
  * in new conversations, perform a search that returns a
    course;
  * click on the course result and verify that its results
    are displayed in place of the original search results
    and not in addition to those results.

Change-Id: Ie33b09fd6ae08969b225a54bbbddeb2161857828
Reviewed-on: https://gerrit.instructure.com/29016
Reviewed-by: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>