don't force MFA reconfig after successful CAS login when MFA is needed

fixes CNVS-13348

the problem was CAS login uses a GET, so we were taking the reconfig
path. so check more specifically for the reconfigure action

test plan:
 * configure MFA for an account
 * make sure you can log in, but *don't* remember the device for
   MFA purposes
 * configure CAS on the account
 * login with CAS
 * it should ask for your MFA code, not to reconfigure MFA

Change-Id: If955ee3ed48456c64c9732fa90c7fb911e1c402d
Reviewed-on: https://gerrit.instructure.com/35668
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>