hide selected message on conversations search
hide selected message on conversations search

fixes CNVS-9140

when searching in new conversations, any selected
message now disappears from the right pane.

test plan:
  * navigate to new conversations as a user with more
    than one message thread;
  * select a message thread in the left pane and verify
    that it loads in the right pane;
  * using the "search by user" textbox in the header,
    search for a user in another thread;
  * verify that the current message in the right pane is
    removed and that you can properly select a message in
    the filtered results in the left pane.

Change-Id: I0d51cb3f858c1f21ff09fdc57d0e47d4e9329acd
Reviewed-on: https://gerrit.instructure.com/26540
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Jon Willesen <eb5ce5e6a76c243326fecfa94534101600f87bd9@instructure.com>
QA-Review: Steven Shepherd <e794458448443812fb2685675aab3bbd1a3891bc@instructure.com>
Product-Review: Zach Pendleton <514dbb4d54d4f29e454990df7d075678f43f92aa@instructure.com>

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