Add Sourcegraph feature flag yml and ref to `Feature.enabled?` [RUN ALL RSPEC]

What does this MR do?

This is a second swing at !49442 (merged) which failed because a CI job failed to detect the feature flag reference because it only used Feature.get (for some UX requirements reasons).

Worst Case Analysis

How do we know this won't break master again?

I titled this MR with [RUN ALL RSPEC] so we can run the specific job that failed, and in this MR it passes https://gitlab.com/gitlab-org/gitlab/-/jobs/917180804 🎉

How do we know this doesn't break the Sourcegraph feature?

The changes to the production code are well covered by the sourcegraph_spec.rb.