Add audio/webm to Special:TimedMediaHandler

Doesn't do much right now, as all webm is registered in the database
as video/webm, by MimeMagic.php

Also fix listing of audio formats.

Change-Id: Ibd118a190911fc37ef1f9814e9868c8a1e5645e0
Merge "Add audio/webm to Special:TimedMediaHandler"