conversations: add ctrl-a for conversation list

fixes CNVS-10321

test plan:
  * open conversations
  * press ctrl-a
  * verify that all conversations are selected
  * verify that it works in Chrome and IE
  * verify that when you select all or quickly select and deselect
      conversations in the list, the details pane shows a conversation
      only if it is the only conversation selected

Change-Id: Ib851399dc156d74085378d45e2708b0ed821f78f
Reviewed-on: https://gerrit.instructure.com/28710
Reviewed-by: Joel Hough <joel@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>