Add Document model

Created by: njoyard

This PR moves dossier links to a new Document entity. This will enable having multiple linked documents for each dossier; in addition each document is linked to a chamber. This last point also creates a real link between dossiers and chambers, while until now we only had a link through votes (dossier -> proposals -> votes -> representatives -> mandate -> chamber).

The PR also rewrites FranceData dossier references: those were URLs, and are now a real dossier reference extracted from those URLs.

(coverage decreased mainly because of the added admin page, we do not test admin pages here... yet)