build: use configuration file, fix track duration handling

[mjs]

• build.py now uses classifier.yaml
• tracks_folder moved out of extract section of config. It's used in more than once place.
• build.py now uses track_folder config instead of hardcoded path
• various import fixes
• use a later end date in the build step
• calculate track duration on the fly - it no longer seems to be in the track metadata