It's been possible to create custom release tracks for a while, so we should support these.

Basically it means removing the current track name validation logic, but probably we should move that logic to the point where we're connected to Google Play.

There we can enumerate the list of available tracks, and check that the user has specified a valid one. Or, we could just attempt the operation, and let Google Play do the validation — however that might be annoying, as we could be uploading loads of large files, and only discover that the track name was wrong after uploading, when we attempt to allocate those files to the track.