TODO
- user management
    * ownership of datasets
    * ownership of classifiers
    * access list for datasets
    * access list for classifiers
    * voting on multiple labels
    * user agreement
- table views
    * different sort criteria
- use ids rather than names in batch jobs
- cleaning
    * purge unreferenced documents
- versioning
- backup of deleted data
- additional tools:
    * estimated accuracy for classifiers