introduce setting "allowAdHocMeetings" to show or hide Adhoc-Meetings button

assets/config.json

@@ -2,6 +2,7 @@
   "jitsiLauncher": "meet.bnvb.eu",
   "jitsiHost": "meet.bnvb-ingenieure.de",
   "nextcloudBase": "https://cloud.bnvb-ingenieure.de",
+  "allowAdhocMeetings": true,
   "launchTimeout": 2000,
   "backgroundImage": "assets/images/background.png",
   "logo": "assets/images/logo.svg",
@@ -11,5 +12,6 @@
     "linux": "https://github.com/jitsi/jitsi-meet-electron/releases",
     "ios": "https://apps.apple.com/app/jitsi-meet/id1165103905",
     "android": "https://play.google.com/store/apps/details?id=org.jitsi.meet"
-  }
+  },
+  "allowAdhocMeetings": true
 }