Record serializers need to include a link to the bucket

See how to create a record and a file: https://github.com/inveniosoftware/inveniosoftware.org/issues/106

Currently when you create a record the response looks like this:

```json
{
"created": "2019-08-01T09:21:47.261207+00:00",
"id": "2",
"links": {
"self": "https://127.0.0.1:5000/api/records/2"
},
"metadata": {
"contributors": [
{
"name": "Doe, John"
}
],
"id": "2",
"title": "Some title"
},
"revision": 0,
"updated": "2019-08-01T09:21:47.261214+00:00"
}
```

It should look similar to this:

```json
{
"created": "2019-08-01T09:21:47.261207+00:00",
"id": "2",
"links": {
"self": "https://127.0.0.1:5000/api/records/2",
"files": "https://127.0.0.1:5000/api/records/2/files/"
},
"metadata": {
"contributors": [
{
"name": "Doe, John"
}
],
"id": "2",
"title": "Some title"
},
"revision": 0,
"updated": "2019-08-01T09:21:47.261214+00:00"
}
```

Adding the ``links.files`` properties.