Update document

[aspino]

• [x] Check that the updated document is signed with the same key as the original owner
• [x] Set up master/administrative server keys that override all permissions and can update/delete any document
• [x] Implement document versioning
• [x] Allow setting a param to enable a more permissive document creation. When this param is set, if the user tries to create a resource that has already been created, it will be silently updated instead of raising an error.