Remove Uris from REST API

Currently, REST API endpoints for users, groups and content will return a number of `Uri`s (e.g. `largeUri`, `mediumUri`, `smallUri`, `thumbnailUri`, etc.) when the entity has a profile picture. These should not be part of the response, as they can be confusing for people using the REST API directly.

e.g.

```
{
    "tenant": {
        "alias": "cam",
        "displayName": "University of Cambridge"
    },
    "id": "c:cam:xJWYxPdRI",
    "visibility": "private",
    "displayName": "Island-picture.jpg",
    "description": "",
    "resourceSubType": "file",
    "createdBy": {
        "tenant": {
            "alias": "cam",
            "displayName": "University of Cambridge"
        },
        "id": "u:cam:xkz4o2gvIX",
        "displayName": "Bert Pareyn",
        "visibility": "public",
        "picture": {
            "smallUri": "local:u/cam/xk/z4/o2/gv/xkz4o2gvIX/profilepictures/small.jpg",
            "mediumUri": "local:u/cam/xk/z4/o2/gv/xkz4o2gvIX/profilepictures/medium.jpg",
            "largeUri": "local:u/cam/xk/z4/o2/gv/xkz4o2gvIX/profilepictures/large.jpg",
            "small": "/api/download/signed?uri=local:u/cam/xk/z4/o2/gv/xkz4o2gvIX/profilepictures/small.jpg&signature=fda46a8323a3fa283af377d6a3e0bede57172ef9&expires=1387729800000",
            "medium": "/api/download/signed?uri=local:u/cam/xk/z4/o2/gv/xkz4o2gvIX/profilepictures/medium.jpg&signature=3aa6c274d16497d089b4848fb3085cb1109f0bb9&expires=1387729800000",
            "large": "/api/download/signed?uri=local:u/cam/xk/z4/o2/gv/xkz4o2gvIX/profilepictures/large.jpg&signature=5a416d6062572e4322e1cad8343c0d41e2f10d04&expires=1387729800000"
        },
        "profilePath": "/user/cam/xkz4o2gvIX",
        "resourceType": "user",
        "acceptedTC": 1386694519756,
        "lastModified": "1387727690664"
    },
    "created": "1387728538742",
    "lastModified": "1387728538744",
    "downloadPath": "/api/content/c:cam:xJWYxPdRI/download/rev:cam:eJlZKevO0I",
    "profilePath": "/content/cam/xJWYxPdRI",
    "resourceType": "content",
    "latestRevisionId": "rev:cam:eJlZKevO0I",
    "filename": "Island-picture.jpg",
    "size": 128319,
    "mime": "image/jpeg",
    "previews": {
        "largeUri": "local:c/cam/xJ/WY/xP/dR/xJWYxPdRI/previews/rev-cam-eJlZKevO0I/large.jpg",
        "mediumUri": "local:c/cam/xJ/WY/xP/dR/xJWYxPdRI/previews/rev-cam-eJlZKevO0I/medium.jpg",
        "smallUri": "local:c/cam/xJ/WY/xP/dR/xJWYxPdRI/previews/rev-cam-eJlZKevO0I/small.jpg",
        "status": "done",
        "total": 4,
        "thumbnailUri": "local:c/cam/xJ/WY/xP/dR/xJWYxPdRI/previews/rev-cam-eJlZKevO0I/thumbnail.jpg",
        "thumbnailUrl": "/api/download/signed?uri=local:c/cam/xJ/WY/xP/dR/xJWYxPdRI/previews/rev-cam-eJlZKevO0I/thumbnail.jpg&signature=49b9032e0771a7f9bed283683646a95ab65d2c88&expires=1387729800000",
        "smallUrl": "/api/download/signed?uri=local:c/cam/xJ/WY/xP/dR/xJWYxPdRI/previews/rev-cam-eJlZKevO0I/small.jpg&signature=20e42afe2d1753314e70342f227c31d659b3becd&expires=1387729800000",
        "mediumUrl": "/api/download/signed?uri=local:c/cam/xJ/WY/xP/dR/xJWYxPdRI/previews/rev-cam-eJlZKevO0I/medium.jpg&signature=69ac05d9a98124f6222a840e5c3177a878e41db0&expires=1387729800000",
        "largeUrl": "/api/download/signed?uri=local:c/cam/xJ/WY/xP/dR/xJWYxPdRI/previews/rev-cam-eJlZKevO0I/large.jpg&signature=479fb5911f8e011d1dacca287581e8ac81fa56aa&expires=1387729800000"
    },
    "signature": {
        "signature": "acd391e7c5cde8ccf887237b65b00c2d6f931f79",
        "expires": 1387729800000,
        "lastModified": "1387728538744"
    },
    "isManager": false,
    "canShare": false
}
```