Site list endpoint return `resource_url` as relative URL

[rafaelsierra]

GET /v1/feeds/sites HTTP/1.1

{
    "count": 1,
    "next": null,
    "previous": null,
    "results": [{
        "id": 1,
        "resource_url": "/v1/feeds/sites/1",
        "url": "https://www.reddit.com/",
        "feed_url": "https://www.reddit.com/.rss",
        "title": "reddit: the front page of the internet",
        "last_update": "2016-12-10T21:39:38.771085Z"
    }]
}

Return should be an absolute URL