use only completed promotion usages when checking usage limit

It should only count completed orders against the usage_limit.  Counting incomplete orders as using a coupon doesn't make sense.  We could also probably take the promotable argument off of the usage_limit_exceeded? method signature but I wasn't sure that was a good idea.
If you bring load_order into a before filter, for this action, and other member actions, you can add another before filter for the coupon handling after that one. We don't need the authorize! (I believe) because no matter what if promotions are ineligible, they should be removed when noticed at all times.