[migrate] Add "global" field to events

Currently no forms will input this, and I'm going to do that right now.
Adding rudimentary DB inputs for the global flag.

Everything works now, but I still need to bulletproof it.
Uniquify all events in a prettier way.
Add "global?" function to event, to make stuff easier.
Don't allow the user to join or leave an event if the event is global.

That would be rather pointless, because they'll see it anyways.
Some changes to the event#show's information output.