Render Rapid Diffs on the MR creation page

What does this MR do and why?

For #500269 (closed)

Enables the ?rapid_diffs=true option on the MR creation page, which swaps the UI from our existing Diffs app to the Rapid Diffs app.

MR acceptance checklist

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the rapid_diffs feature flag: rails c => Feature.enable( :rapid_diffs )
2. Start the process to create a new MR
   • Choose the source branch, and continue to the final step
3. 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
4. Note that the "Changes" tab below the main form is now rendering the Rapid Diffs implementation

Caveats

Note that the sidebar (file tree) does not work because there's no metadata endpoint, which isn't present because:

1. It originally was not available
2. Updating the back end to use it didn't work for me: #525429 (closed)