add conditional feature flag on quiz stats

closes CNVS-14678
This commit allows the quiz stats feature flag to be registered only when
within the beta environment (or development or test). It should not be available in production.

Test plan
- Deploy normally to your QA portal.  The 'quiz stats' feature flag should
  be available and operational.
- Remove the testcluster.yml file from your config/ directory (or move
  / back it up to a safe place)
- Restart your QA portal.
- The quiz stats feature flag should not be available or visible.

Change-Id: If51af6bae18b31fed8f7b2d666f59562ad0fb18e
Reviewed-on: https://gerrit.instructure.com/39330
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>