Fix fullcalendar v5 options for background events

`rendering` was used in v4, in v5 `display` changes the way how events
are rendered. This caused blocks in the UI that then made the free/busy
schedule grow vertically and so became less useful.

Ref https://fullcalendar.io/docs/upgrading-from-v4
Ref https://fullcalendar.io/docs/background-events



Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>