Multi-event capabilities

[opatut]

This is a big one. Attach everything to events and then filter by that event to display e.g. statistics.

Probably some things should not be related to events, e.g. product groups should be able to display items in the dashboard from different events, such that we can just continue to use the previous years' data and extend it.

[felixrindt]

As you said in the DM, to keep product types like Shirt/Hoodie and colors etc. over multiple events we might need to touch the product models too.
Prices would need to be set per event etc.