communication channel confirmation_url param for users api

fixes CAT-422

Test Plan
  1. With an admin user, submit a POST request to the canvas api
     to create a new user with the [:communication_channel][:confirmation_url]
     param set to true
  2. Ensure that the json payload returned contains a valid [:confirmation_url]
     value which can be used to complete the user registration on canvas
  3. Ensure that users outside site-admins and account-admins cannot
     access this data via the api

Change-Id: I6566202c12241893590bb98705e28287b1fa76fc
Reviewed-on: https://gerrit.instructure.com/40851
Reviewed-by: Dave Donahue <ddonahue@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Adam Phillipps <adam@instructure.com>
QA-Review: Adam Phillipps <adam@instructure.com>