AKOnline: Fix virtual room admin (url editable again)

Due to inheritance issues and the integration of the availability widget it was no longer possible to edit virtual room's urls. This resolves this.

AKOnline/admin.py

 from django.contrib import admin
 
-from AKModel.admin import RoomAdmin
+from AKModel.admin import RoomAdmin, RoomForm
 from AKOnline.models import VirtualRoom
 
 
+class VirtualRoomForm(RoomForm):
+    class Meta(RoomForm.Meta):
+        model = VirtualRoom
+        fields = ['name',
+                  'location',
+                  'url',
+                  'capacity',
+                  'properties',
+                  'event',
+                  ]
+
+
 @admin.register(VirtualRoom)
 class VirtualRoomAdmin(RoomAdmin):
     model = VirtualRoom
+
+    def get_form(self, request, obj=None, change=False, **kwargs):
+        if obj is not None:
+            return VirtualRoomForm
+        return super().get_form(request, obj, change, **kwargs)