conversations: only poll unread count on inbox page

fixes CNVS-14355

test plan:
  * open conversations
  * mark a conversation read or unread
  * wait 30 seconds
  * verify that your unread message count is updated

  * open your browser's network monitor
  * switch to another tab
  * wait 30 seconds
  * verify that /api/v1/conversations/unread_count was not hit

  * open a different canvas page
  * wait 30 seconds
  * verify that /api/v1/conversations/unread_count was not hit

Change-Id: Ib33ac1fda7e16717508cfb34bf360d8b917468d1
Reviewed-on: https://gerrit.instructure.com/37954
Reviewed-by: Brian Palmer <brianp@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>