A way to upload backing tracks

[gwillen]

Would be good to have a way to upload backing tracks through the web interface. Ideally handling all the conversion steps automatically, and renaming the file to the specified target name.

[jeffkaufman]

Alternatively, perhaps they should be ephemeral, similar to lyrics.

[jeffkaufman]

Here's an initial version:

• Add a new service, responsible for handling backing track uploads
• Have nginx proxy_pass /upload to it
• The upload service reads the bytes from the request, decodes any compression, and writes to audio/User Upload

Later on, we can save the original name to audio/user_upload_fname.txt or something, pass that to clients, and special case it in backing track selection.