List usernames in the registry.

GET /api/user-list
HTTP 200 OK
Allow: GET, HEAD, OPTIONS
Content-Type: application/json
Vary: Accept

[
    {
        "username": "CBS"
    },
    {
        "username": "removed_hack01"
    },
    {
        "username": "Institut Pasteur"
    },
    {
        "username": "GenOuest"
    },
    {
        "username": "removed_hack04"
    },
    {
        "username": "PSB"
    },
    {
        "username": "removed_BioModels"
    },
    {
        "username": "ELIXIR-SI"
    },
    {
        "username": "ELIXIR-EE"
    },
    {
        "username": "BioJS"
    }
]