✨(models/api) add link access reach and role

Link access was either public or private and was only allowing readers.

This commit makes link access more powerful:
- link reach can be private (users need to obtain specific access by
  document's administrators), restricted (any authenticated user) or
  public (anybody including anonymous users)
- link role can be reader or editor.

It is thus now possible to give editor access to an anonymous user or
any authenticated user.

src/backend/core/api/permissions.py

@@ -62,6 +62,9 @@ class IsOwnedOrPublic(IsAuthenticated):
 class AccessPermission(permissions.BasePermission):
     """Permission class for access objects."""
 
+    def has_permission(self, request, view):
+        return request.user.is_authenticated or view.action != "create"
+
     def has_object_permission(self, request, view, obj):
         """Check permission for a given object."""
         abilities = obj.get_abilities(request.user)