force gradebook transition

this commit forces on the new, accessible, individual gradebook view and forces
of the old GB1

closes CNVS-12327

test plan:
- make sure that individual gradebook view is on and GB1 is off and that you
  can't change it from the UI as a teacher or site admin
- now run the following (to simulate not already having the flag set to on):

  FeatureFlag.where(feature: "screenreader_gradebook").destroy_all

- check again to make sure it is on and can't be turned off

Change-Id: I23cb476d051b4e7697de47432dbe4a408b9800ef
Reviewed-on: https://gerrit.instructure.com/35464
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>