Scaffold group secrets manager page

What does this MR do and why?

This adds the route for the group secrets manager page. The page uses the same Vue components used for the project secrets manager, and it will serve as the foundation for starting the work for group secrets.

The page is served in /groups/<group-name>/-/secrets.

I renamed the entity prop to context since I think this better describes what it's for. It's likely we'll have several contexts other than group or project e.g. instance or organization secrets, or perhaps a "context-less" secrets manager.

References

Resolves #577451 (closed).

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Upload a Premium+ license.
2. Enable the feature flag secrets_manager_group.
3. Go to /groups/<group-name>/-/secrets and verify that the secrets page is up.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.