Try and get fixed-bitrate audio seeking working

[alastair]

Theoretically we should be able to seek into the middle of a file without it being 100% loaded if it's fixed-bitrate and we send the correct headers. Work out how to do this.

[alastair]

Some technical notes in soundmanager docs: http://www.schillmania.com/projects/soundmanager2/doc/technotes/#byte-serving

[alastair]

If people are listening to audio, we don't want them to be able to download it very easily. Restrict the speed to the bitrate of the audio.

At the same time, we need un-restricted access for people who have API access to the audio. It should be unrestricted if someone from staff is logged in, or they have a correct downloader auth token set.

[alastair]

Not all browsers support byte serving very well. Current testing shows that IE 9 and 10 work with MP3. Firefox works with ogg and wav. Mp3 support is in Nightly, due to be released as Fx33 in October. Chrome and Safari is unknown, but it looks like neither windows nor Linux can do byte serving with mp3s.

[alastair]

This is needed for @sankalpg very soon for his interfaces