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-180
HTTP 200 OK
Allow: GET, HEAD, OPTIONS
Cache-Control: max-age=0, no-cache, no-store, must-revalidate
Content-Type: application/json
Expires: Sun, 16 Jun 2019 16:49:27 GMT
Vary: Accept

{
    "count": 39,
    "next": null,
    "previous": null,
    "results": [
        {
            "source": "direct-sharing-180",
            "name": "Genetics of Personality Type",
            "usernames": [
                "madprime",
                "gsdcomo",
                "emeadows317",
                "benc",
                "taximadman",
                "stephenbrad",
                "laurenshearer",
                "dencook",
                "kdgreis",
                "M_P",
                "dave2019",
                "personalitygenie",
                "tad",
                "Ronin",
                "deaden",
                "Laurenshearer86",
                "Cat",
                "smallstepstoday",
                "Neat",
                "Robin",
                "Pgtest",
                "zansewek",
                "ricardochavira",
                "bkothe",
                "Suzichains"
            ]
        }
    ]
}