Response to 'conferences' requests through the API doesn't include url nor join_url

[jfederico]

Summary:

Steps to reproduce:

Expected behavior:

Additional notes:

 "conferences": [{
        "id": "109",
        "title": "garrett test join_url",
        "conference_type": "BigBlueButton",
        "conference_key": "instructure_web_conference_3NrpER9YDi57iQ51ZqWy2d4WlHRdPpKn0zd9ejpg",
        "started_at": "2018-06-28T15:53:12Z",
        "description": "",
        "duration": null,
        "ended_at": null,
        "has_advanced_settings": false,
        "long_running": true,
        "user_settings": {
            "record": true
        },
        "recordings": [],
        "url": null,
        "join_url": null,
        "users": [9, 6, 12, 8, 11, 13, 1, 21, 20, 19, 17]
    }