external tool logout service

test plan:
 0. configure an external tool that
  - reads the $Canvas.logoutService.url
  - POSTs to this address, adding a 'callback' parameter
    specifying the URL canvas should GET when the user
    logs out of Canvas
  (see the platform team for help with this step)
 1. launch this tool
 2. log out of canvas
 3. ensure the logout callback is called

closes CNVS-15764

Change-Id: Iaa66212d8068f2c6f919477bae509a05f1a2993c
Reviewed-on: https://gerrit.instructure.com/41745
Reviewed-by: Brad Humphrey <brad@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>