Use init.json properties for "Showing feedback button"

[michalharakal]

Rules to show feedback:

• after talk has been started
• feedback feature avaliable for the conference
• user logged in ???
• showed only during the span of timeSlotVisible Minutes

Sample data for Javaland 2019:

{
    "id": "javaland2019",
    "name": "JavaLand 2019",
    "year": "2019",
    "url": "http://javaland.dukecon.org/2019",
    "homeUrl": "http://www.javaland.eu",
    "homeTitle": null,
    "imprint": {
        "de": "https://www.javaland.eu/de/impressum/",
        "en": "https://www.javaland.eu/en/imprint/"
    },
    "termsOfUse": {
        "de": "https://www.javaland.eu/de/nutzungsbedingungen/",
        "en": "https://www.javaland.eu/en/term-of-use/"
    },
    "privacy": {
        "de": "https://www.javaland.eu/de/datenschutz/",
        "en": "https://www.javaland.eu/en/privacy/"
    },
    "startDate": "2019-03-19",
    "endDate": "2019-03-21",
    "authEnabled": true,
    "admin": "../rest/admin/javaland2019",
    "forceUpdate": "../rest/conferences/update/javaland2019",
    "conferences": "../rest/conferences/javaland2019",
    "events": "../rest/eventsBooking/javaland2019",
    "keycloak": "../rest/keycloak.json",
    "feedbackServer": {
        "active": true,
        "timeSlotVisible": 10080
    }
}