conversations: recompute unread count when opening inbox

test plan:
  * set your unread conversations count to an incorrect value:

u = User.find(X); u.unread_conversations_count = 20; u.save

  * open Canvas
  * open your inbox
  * verify that your unread count badge is correct

Change-Id: I11b504dd9ee78db105de88305827d39e9d8433d3
Reviewed-on: https://gerrit.instructure.com/36707
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>