Change Document.submitter from a string to a json / dict / mapping

MANUAL-UPGRADE: Change column type of document submitter to json in
postgres db
Create BaseContent as a base class for Document and Binder
Add authors field in metadata and set it to the initial submitter

MANUAL-UPGRADE: Add authors field in document in postgres db
Add RoleRequest model and memory and postgresql storage

MANUAL-UPGRADE: Add rolerequest table in postgres db
Add role requests when new authors are added
Add /roles/{id}@draft.json for accepting roles