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


git-svn-id: https://develop.svn.wordpress.org/trunk@26467 602fd350-edb4-49c9-b593-d223f7449a82
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


git-svn-id: https://develop.svn.wordpress.org/trunk@26468 602fd350-edb4-49c9-b593-d223f7449a82