Add webm to converters and set as preferred source

Note that we still need to convert existing videos, which is probably a
day or two of hardcore processing on the server. This simply adds all
the support required to use webm for new videos.

We can push this **after ffmpeg has been recompiled to support libvpx on
the server**. Existing videos needn't have a webm source to work at that
point - the browser will fallback to one of the other available formats.

To convert the existing videos, use a script like this:

    #!/bin/bash
    for f in *.mp4
    do
        bn=$(basename "$f" ".mp4")
        if [ -e "$bn.png" ]
        then
            echo "Skipping $bn"
        else
            ffmpeg -i "$bn.mp4" -c:v libvpx -c:a libvorbis -quality good -b:v 1M -crf 5 "$bn.webm"
        fi
    done