Return neighbor USs when fetching an US

The response for an User Story now contains:

{
    ...
    "neighbors": {
        "previous": {
            "id": ...,
            "ref": ...,
            "subject": ...
        },
        "next": {
            ...
        }
    }
}

If there's a neighbor missing the response will contain:

{
    ...
    "neighbors": {
        "previous": {},
        ...
    }
}