🐛(backend) fix link definition select options linked to ancestors

We were returning too many select options for the restricted link reach:
- when the "restricted" reach is an option (key present in the returned
  dictionary), the possible values for link roles are now always None to
  make it clearer that they don't matter and no select box should be
  shown for roles.
- Never propose "restricted" as option for link reach when the ancestors
  already offer a public access. Indeed, restricted/editor was shown when
  the ancestors had public/read access. The logic was to propose editor
  role on more restricted reaches... but this does not make sense for
  restricted since the role does is not taken into account for this reach.
  Roles are set by each access line assign to users/teams.

CHANGELOG.md

@@ -132,6 +132,7 @@ and this project adheres to
 
 ## Fixed
 
+- 🐛(backend) fix link definition select options linked to ancestors #846
 - 🐛(back) validate document content in serializer #822
 - 🐛(frontend) fix selection click past end of content #840