Add feature flag to disable the severity override feature at the root group

We need to implement a feature flag that will disable the severity override feature at the root group level.

This feature flag would be around for one Fiscal quarter, and would be removed in %18.2, or %18.3.

This will give time for groups to adjust to this new capability.

Docs for selectively disabling: https://docs.gitlab.com/development/feature_flags/controls/#selectively-disable-by-actor