conversations: clear unread_conversations_count cache

test plan:
  * open Canvas as a user with unread conversations
  * POST to /api/v1/conversations/mark_all_as_read
  * refresh the page
  * verify that your unread conversations badge is gone
  * verify that receiving new conversations also
      immediately updates the badge

Change-Id: Ib0b629ba84a59af102810a415d5955f7a2f4e46a
Reviewed-on: https://gerrit.instructure.com/36698
Product-Review: Braden Anderson <braden@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>