Add license field to SourceFile

[alastair]

We want to be able to filter for only recordings which are available under a certain license. To do, this add a field license to the docserver.SourceFile model.

This should be foreign key to a License model which allows us to add a new license when needed. When uploading new source files we will have to be able to specify a license (either through the dunya dashboard, or the upload API).

Add functionality to the docserver API to allow filtering all recordings that have a particular license