Themes: Use _.throttle() instead of a custom one. Props matveb. Fixes #25981

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Themes: Add a clearing <br/> to the end of .theme-browser.
This is needed to cause the browser to render .theme-browser's height as it's true height, previously the reported height was 1*.theme which caused the Theme scroller to kick in immediately (as the threshold was less than 0).
See #25981 See #25948

Built from https://develop.svn.wordpress.org/trunk@26468


git-svn-id: http://core.svn.wordpress.org/trunk@26366 1a063a9b-81f0-0310-95a4-ce76da25c4cd