Changes the "Cancel" and "Update" controls in buttons for better semantics and
accessibility. On cancel and successful saving, moves focus back to the term title
to avoid a focus loss. Dispatches error and success messages to `wp.a11y.speak`
to give assistive technologies users an audible feedback.
Patch prepared at #wpcdit, first Italian WordPress Contributor Day.
Props garusky, chiara_09.
Fixes#34613.
Built from https://develop.svn.wordpress.org/trunk@35605
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