Add `aria-labelledby` and `aria-describedby` attributes to revision slider selection handle. Add keyboard `:focus` state to revision slider handle. Move 'multiple revision' checkbox before the buttons panel, matching its visual order. Change diff `Title` and `Content` headings from `h3` to `h2` to correct headings hierarchy.
Props joedolson, rishishah, sarahricker, williamalexander, afercia.
Fixes#52303.
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