Make loaddata command more useful

This changes allows us to make more use of the loaddata command.

We know can specify that we want to update data, based on some different criteria then the primary key, allowing us to write something like:

[
  {
    "fields": {
        "name": "GFZ",
        "url": "https://rz-vm65-dev.gfz-potsdam.de"
    },
    "model": "TsmEndpoint",
    "unique": ["name"]
  }
]

This allows us to overwrite the url value for the entry with name = GFZ.