Disable 'unregister' after deadline

We use registrations for events for which people need to pay a fee to participate. However, due to the way the registrations module currently handle's the "unregister" functionality people can unregister even after the deadline and there will be no trace of what they were registered for.

The alternative is to simply disable the possibility to unregister altogether but that gives us different problems. The related issue #2514268: Cancel vs. delete registration provides a solution for making a non-destructive unregister function with rules but it's not really a scalable solution and it might still allow people to change their state to "unregistered" after they have partiicpated in the event, wrecking the bookkeeping.

A way is needed to specify a "cancelled" state and to specify who can set his/her registration to that state and up to what date they can do that (e.g. up to the registration deadline or a separate date).