Refactors Highlander

Created by: Exxion

Assuming I didn't fuck anything up and that there isn't any SCOTLAND-related stuff on the secret repo that this might conflict with, the only functional difference is that it now asks you what type of THERE CAN BE ONLY ONE event you want to start, rather than just immediately starting Highlander. The only two options at the moment are Cancel (which cancels it, of course) and Highlander. I was going to add Wizard Wars along with this but decided I'd rather not have people complain about it not being atomic.

Now there are datums for Highlander-like events, which contain four procs. One is called as the event setup starts and cancels the event if it returns false, one checks if a mob is eligible to become a Highlander, etc., one handles actually turning the mobs into Highlanders, etc., and one is called after all those. Specifics are given in comments above each proc in the base type definition.

The code for actually turning players into Highlanders is mostly unchanged (just moved around), and the rest of the code should be functionally equivalent to how it worked before.

Tested about as fully as I could test it with only one client.