This will allow us to simplify the get_abilities method. It is also
more efficient because we have computed this definitions dict and
the the get_select_options method was doing the conversion again.
We are going to need to compare choices to materialize the fact that
choices are ordered. For example an admin role is higer than an
editor role but lower than an owner role.
We will need this to compute the reach and role resulting from all
the document accesses (resp. link accesses) assigned on a document's
ancestors.