improve nextcloud Jitsi app

Jitsi app works well and is available in faircloud for testing for fairkom staff.

Sources https://github.com/nextcloud/jitsi/

Settings

when entering https://meet.jit.si/ it says "It is highly recommended to set up a dedicated Jitsi instance " - so why not offer list of public instances

and meet.jit.si now requires authentication and since May 2023 Jitsi meetings in iFrames are limited to 5 minutes - they want you to use JaaS https://git.fairkom.net/operations/sales/-/issues/46.

JSON Web Token JWT Secret (optional) - not suitable for jaas yet see https://github.com/nextcloud/jitsi/issues/21#issuecomment-2067963423