Add compliance metadata

• Add field has_pii: We can set this to true if we suspect a document contains Personally Identifiable Information (PII), i.e., likely specially protected under GDPR
• Add field is_public_domain: We can set this to true if we suspect the document is in the public domain, i.e., we don't have to worry about copyright

What needs to be done

• Add values for every document we currently have for a preliminary analysis of our compliance status
• Discuss this preliminary analysis in the group (if nobody objects, we can start working with these judgements as base)