Better support for event-shaped puzzles

[ebroder]

There are a few aspects of events (and related things, like scheduled interactions with Hunt HQ) that aren't handled by Jolly Roger super well today:

• Jolly Roger doesn't have any explicit puzzle checkout/assignment process (a la our previous software). That's generally for the better, but for events you want an explicit claim process.
• Events and interactions generally happen at a pre-scheduled time. It would be neat to have the ability to set some sort of time-based reminder, either for the people that claimed the event or for operators ("make sure X has left for Y").
• Frequently events and interactions can be completed/solved without resulting in an explicit answer. Today, Jolly Roger defines whether a puzzle is "solved" by "is there a non-empty answer". It'd be nice to have an explicit way to track this (in the past, we've done this by just setting the answer for the event to DUMMYEVENTANSWER or whatever)

These features probably don't want to be tied to each other (e.g. you probably want to be able to tag a puzzle as completed-but-with-no-answer even if it doesn't have explicit claims), but it may also be OK if they are all tied to some idea of an "event puzzle".

[ebroder]

Note that having explicit checkout/checkin for puzzles would also be useful for instruction puzzles ("SEND US AN X"), so this might pair nicely with #231, especially if you could turn it on/off on a puzzle that already exists.