outcome gradebook: unhide feature flag

fixes CNVS-11284

test plan:
  * open site admin account feature options
  * verify that outcome gradebook is listed without "hidden" or "development"
  * verify that you can enable outcome gradebook without site admin changes

Change-Id: Id7d14692c6481a2f7c15bd066b07e892df8eddbd
Reviewed-on: https://gerrit.instructure.com/30306
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>