Allow CfP events to happen multiple times

[russss]

Sometimes the same event happens multiple times. We currently deal with this by creating multiple events. Ideally we should allow the same event to be scheduled multiple times.

[Kunsi]

If those events need to be streamed and/or recorded, we must make sure they are uniquely identified (e. g. atleast slug and uuid must be unique, otherwise other tools (especially media.ccc.de) get confused what is what.

[russss]

Good point. I assume the events which occur multiple times are probably not going to be recorded, but for recordings it's probably worth using the ID of the "occurrence"?

(I just wanted to go camping, how have we ended up with such a complex data model??)