WIP on audio: how to create m3u URLs *correctly*

This proof of concept shows how to, given the (bare) filename of an
MP3 file, produce an .m3u file that is completely autogenerated in
Javascript (so that no server round-trip needs to occur) that can
then be downloaded by the user's browser and processed by whatever
program his system has associated with .m3u files (or audio/x-mpegurl
MIME types). The tricky part is that Angular automatically marks
data: URLs as unsafe unless they match a whitelist, but we don't
want to arbitrarily add 'data:' to the whitelist as that could open
a security hole if malicious Javascript gets injected. So we calculate
an appropriate data: URL fragment ("protocol://server.name/audio/"
encoded as base64), and add only that fragment to Angular's whitelist.
Thus, we can serve up .m3u files as data: URLs safely, without opening
any security holes.

Next step: turning the below into a directive that, given an MP3
filename, will construct appropriate "download" and "play" URLs.