Provenance extension

This branch addresses the goals of task #207 (on Kanboard). The aims of the task were to improve the information of the provenance field in our metadata schema. The following goals were defined:

• minimal logical schema for data source metadata (see provenance.toml),
• physical storage of data source metadata (see provenance.toml),
• HTTP endpoints for human readable page and JSON API for data source metadata (check on http://localhost:8081/provenance/<id>),
• linking of appropriate provenance paths on the dataset detail page (check on http://localhost:8081/dataset/<source>/<id>).