add initial feature flags needed to kick off verifiers project (pt.1)

flag: file_association_access
flag: disable_file_verifier_access

refs: GROW-20
Change-Id: I10247cad033001240429c9ee3cb2c5d24450b017
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/370077


Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Mysti Lilla <mysti@instructure.com>
QA-Review: Kasra Rismanchi <kasra.rismanchi@instructure.com>
Product-Review: Kasra Rismanchi <kasra.rismanchi@instructure.com>

config/feature_flags/verifiers.yml

+---
+file_association_access:
+  state: hidden
+  applies_to: RootAccount
+  display_name: File Association Access
+  description: Allow users to access files associated with a course
+disable_file_verifier_access:
+  state: hidden
+  applies_to: RootAccount
+  display_name: Disable File Verifier Access
+  description: Disable access to the file verifier through UUID