refresh CAS ticket in find_user

fixes: CNVS-16438

Found an issue where the refresh_cas_ticket method was being called
before find_user which will fail since @pseudonym_session would not have
been populated yet.  This removes that method from application_controller
and adds the refresh to the CAS expired check.

Test Plan:

  - Setup CAS with Canvas
  - Log in with CAS and take note of the ticket that was used.
  - Open up redis and find a record with a key of cas_ticket:#{ticket}
    from the previous step.
  - The note the ttl of the record in redis.
  - Refresh the page and the ttl should have been extended.
  - Do the same for an ajax call.  The ttl should have been extended.

Change-Id: Ieb25f05765fed2cfb3de00ed6a49966fd3d04698
Reviewed-on: https://gerrit.instructure.com/44062
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Nick Cloward <ncloward@instructure.com>