Improves the `containFocus()` function to always get the correct first and last
focusable elements, even when the theme browser shows the active theme details.
Also, when on the first and last theme, adds a `disabled` attribute on the
previous and next navigation buttons to make them not focusable.
Fixes#37383.
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git-svn-id: http://core.svn.wordpress.org/trunk@38025 1a063a9b-81f0-0310-95a4-ce76da25c4cd