hide srgb behind a feature flag

fixes CNVS-10336

test plan
 - with Screenreader Gradebook not enabled
 - navigate to srgb and it should redirect you to gb2
 - enable Screenreader Gradebook on the course settings page
 - navigate to srgb and it should load

Change-Id: I106ec2093693215f3ed74db61f835fa8e996831a
Reviewed-on: https://gerrit.instructure.com/28400
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>