Add sound "sources" as a field in bulk describe data file (and in API)

[ffont]

The "bulk describe with data file" functionality does not currently support specifying sound sources as you could normally do using the sound describe/edit UI. Support for adding sound sources should be added.

The form used for that already support the field 'sources', but extra code needs to be added to make it work properly, a new column should be added to the csv example files, etc.

Also, we could add support for the sources field in API methods for describing and editing sounds.