Present the tab group for merge requests above the form

What does this MR do and why?

See: #370364 (comment 1069539665)

Prior to this change, users needed to scroll to the bottom of th page to see a summary of what is included in the new merge request.

By placing the tab group above the form (but below the branch chooser), the user can more easily see what the content of the merge request will be.

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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