feat: multi-authorship

I think YouTube is onto something with their new "Collabs" feature. I thought, "well, writers sometimes don't write alone, plus something like that in Leaflet would be perfect for interview transcripts."

So, here is what I propose - multi-authorship. When one publishes a leaflet, one can send invites to co-authors - publications or individual users - that helped shape this leaflet. While collaborative real-time editing (Zed/Google Docs-style) is off-table for now, multi-authorship will lay the foundation for that.

Upon publishing, the leaflet will be broadcast through all specified co-authors.

Why? It's a great way to discover new publications to read for users and attribute co-authors for Leaflet writers.