[bug] cancelled memberships still show rights if any previous membership held had them

[offbyone]

Because of the additive way rights are calculated in reservations.rb, if a contact had any order that had rights before the current one without, it'll still retain all the rights of those previous memberships. This is important for cases where a membership loses rights if purchased after a certain point -- we need to ensure that our nomination rights are tied to "when" a membership is acquired -- but when we cancel a membership it retains those rights, which is problematic from a hugo admin PoV.

I'm not entirely sure how to solve this in the current model; subtractive rights are probably a bad idea, but we might need to set up a different way of modeling them to make this work. This won't be just a code hack, it'll need DB modeling work too.