session_affects_permissions -> permissions_key

fixes CNVS-13711

test-plan:
 - regression tests of:
   * permissions of admin with pending invitation to a course
   * user that typically would not have access to a portfolio, but
     visited the portfolio with a verifier string earlier in the session
     (but has no verifier string now)
   * access to attachments on the safe file domain

Change-Id: Ie321c77655e6ba5e87fd35a079086a48608f5d0e
Reviewed-on: https://gerrit.instructure.com/38029
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>