Save relationships when creating or updating an item

Exemple request:

```
{
    "active": false,
    "name": "Test alex 12",
    "tags": [31,39]
}
```

(works both for ``create`` and ``update`` actions).
Resulting object:

```
{
    "active": false,
    "tags": [
        {
            "active": true,
            "name": "Artenschutz",
            "id": 31
        },
        {
            "active": false,
            "name": "Auenlandschaft",
            "id": 39
        }
    ],
    "name": "Test alex 12",
    "id": 194
}
```

To remove all links, use
```
tags: []
```