authoring roles and acceptances database tables

roles.sql

create table roles(
  id integer unique sequence,
  document_id uuid,
  role_type role_types,  -- e.g. author, illustrator
  user_id text,  -- username as it appears in accounts
  profile json,  -- a copy of the user's profile json
  has_accepted boolean,
  requester text,
  assignment_data timestamp with timezone default current_timestamp,
  primary key (document_id, role_type, user_id),
  foreign key (document_id) references document (id)
  )