Serialize deletion capabilities on an element

To do https://gitlab.com/arkindex/frontend/-/issues/790 the frontend needs to know if the current user can delete a given element.

The RetrieveElement should have a new property to showcase the current user rights:

• a boolean manageable that indicates if the user can "manage" (ie edit + delete) the element
• a more versatile rights list that could either be:
  • [read]
  • [read, write, admin]