Better handle metadata update

[phoebus-84]

Any registered user can change the metadata of an economic resource. This was necessary to update the metadata of the resource, either at the claim or starring stage. However it can obviously to security problems. It would be appropriate to handle this at the backend: specifying the operations that can be done on the metadata (e.g., add a star or remove a star, i.e., the id of the user who is starring) and not have the frontend handle the logic.