Implement "Edit in single file editor" options menu item [Rapid Diffs][Diff File]

What does this MR do and why?

For #514397 (closed)

Phase	Status
Disclosure dropdown	!182222 (merged)
View @ SHA	!182850 (merged)
Open in Editor	👈🏻 You are here

This change - at its core - adds the "Edit in single-file editor" link to the diff file kebab/ellipsis menu in the new Rapid Diffs app.

To do this, it also creates a slot for the header, which we fill with a header that has the merge request object provided to it.

MR acceptance checklist

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

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Before	After

How to set up and validate locally

1. Enable the rapid_diffs feature flag: rails c => Feature.enable( :rapid_diffs )
2. Have an MR with at least 1 change
3. Visit the MR's Changes tab
4. Add ?rapid_diffs=true to the URL (or &rapid_diffs=true if there are already search parameters)
   • Submit the URL change to reload the page
5. Open the "kebab" (vertical ellipsis "Options") menu
6. Ensure the "Edit in single-file editor" menu item is present, correct, and works properly.
   • You can verify this by performing the same steps but without the rapid_diffs=true URL search parameter, as the page will be master without it.