Return an array where each entry is an object with this form:

{ source: "fitbit", name: "Fitbit", usernames: ["beau", "madprime"] }

GET /api/public-data/members-by-source/?source=direct-sharing-258
HTTP 200 OK
Allow: GET, HEAD, OPTIONS
Cache-Control: max-age=0, no-cache, no-store, must-revalidate
Content-Type: application/json
Expires: Sat, 31 Oct 2020 13:57:01 GMT
Vary: Accept

{
    "count": 50,
    "next": null,
    "previous": null,
    "results": [
        {
            "source": "direct-sharing-258",
            "name": "The Quantified Heart",
            "usernames": [
                "madprime",
                "beau",
                "padams89",
                "gsdcomo",
                "annallull",
                "gedankenstuecke",
                "ramonimo",
                "Dyfflinviking",
                "taximeow",
                "stephenbrad",
                "LizMorgan",
                "Agaricus",
                "saw2112",
                "kdgreis",
                "mako",
                "mikazilla",
                "Iain",
                "yeltsinxyz",
                "Valeria",
                "myrthe07",
                "danilo",
                "hubert",
                "Perico429",
                "R_jn",
                "stephthegeek",
                "matfax"
            ]
        }
    ]
}