Add github_import_gists feature flag

What does this MR do and why?

Wraps #371099 (closed) under feature flag

Screenshots or screen recordings

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How to set up and validate locally

Disable by default, to enable:

From Rails console: Feature.enable(:github_import_gists)

Check POST /api/v4/import/github/gists to return 404 when feature is disabled Check POST /api/v4/import/github/gists to return 202 when feature is enabled

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.