Add ability to create commitment certifications

This depends on gitlab.com:schwalbe-hd/schwalbe#259: As soon as this is implemented, there is a standalone pdf_generator package we can use to generate certifications similar to the ones created by Schwalbe (see example).

This would need several steps:

• Add pdf_generator as dependency
• Create custom Coli CSS modifications
• Create a set of Coli templates (HTML) and corresponding certification classes (or find something dynamic for the latter)
• Document how to create such a template
• Add a converter function similar to the main implementation in Schwalbe

The API should the look like this:

1. Create a JSON file containing one or more certification requests (maybe also just one)
2. Convert this to PDF (or maybe HTML) with the Coliverter
3. Enjoy your beautiful commitment certifications