Restrict audio download rate when streaming

[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. This will require proper seeking support (#68)

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.