Add media support to PiccoloCRUD

[dantownsend]

It would be good if we modified PiccoloCRUD, so it could accept a media_storage argument (like create_admin does in Piccolo Admin).

PiccoloCRUD(
    Movie,
    media_columns=[
        LocalMediaStorage(Movie.poster, media_path='/srv/media/movie_posters/')
    ]
)

We could then add a new GET parameter called something like __media_urls, which then auto adds the URL to the response for accessing the media.

GET /1/?__media_urls
{
    'id': 1,
    'name': 'Star Wars',
    'poster': 'some-file-abc-123.jpg',
    'poster_url': '/media/some-file-abc-123.jpg',
}

Once this is in place, we can refactor Piccolo Admin slightly, so it passes the media_storage argument to PiccoloCRUD, and lets PiccoloCRUD do all of the heavy lifting.