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/?format=api&source=direct-sharing-138
HTTP 200 OK
Allow: GET, HEAD, OPTIONS
Cache-Control: max-age=0, no-cache, no-store, must-revalidate
Content-Type: application/json
Expires: Sun, 21 Jul 2019 06:20:16 GMT
Vary: Accept

{
    "count": 43,
    "next": null,
    "previous": null,
    "results": []
}