BBB: mark recording as permanent

If a BBB meeting or course with BBB meetings is deleted, existing BBB recordings will also be deleted. If an external server like Opencast is used to store the recordings, they will be deleted as well. This is not always desired or only selected recordings should be deleted on the external server.

To distinguish which recordings should be deleted / not deleted on the external server, a flag "permanent Recording" is introduced.

Implementation:

• Implement flag on the BBB recording in OpenOlat to mark it as "permanent" (not deletable)
• Default value in local.properties to mark recordings as permanent: true / false
• If possible: override the default value at the individual recording
• Adapt data model and Opencast handler