[server] RetrieveItem's audio_url field dynamic behavior

[gnuletik]

When retrieving an Item through the API, the audio_src field already includes the mp3 and ogg keys.
#172 should add flac key.

We want to avoid providing a MP3 file encoded with flac because it would be a waste of bandwidth.
So, we expect the flac field to be defined only if the source file is a lossless format.

Some clients could expects a flac file to always be defined (if the client only works with flac).
In order to work around this use-case, we can add a query parameter to the route to force specific encoding.

The URL would look like this:

$ curl /timeside/api/items/f819b56c-e4c4-4762-83ac-77eb81084268/?forceEncoding=flac,mp3,ogg
# Expect audio_url.flac, audio_url.mp3 and audio_url.ogg to be defined

$ curl /timeside/api/items/f819b56c-e4c4-4762-83ac-77eb81084268/?forceEncoding=mp3
# Expect at least audio_url.mp3 to be defined

It would be nice to add some docs about this behavior (probably in the description field of the schema).