Make `<audio>` elements in playlists responsive.

See #27625.


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Call MediaElement's shim method for `setSrc()` when dynamically updating the playing source in playlists, which will properly set the source for mobile.

Make the labels for playlist items slightly smaller to account for size on mobile.

Fixes #27625.


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Theme Installer fixes:
 * Better more filters section. props sonjanyc for some mockups.
 * Better handling of no results.
 * Use # for hrefs.

props matveb.
see #27055.

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TinyMCE: update to 4.0.21, see #24067
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