Fix promo code validation

Because group promo codes actually change to individual promo codes once you claim them, our validation for not letting discounted badges use promo codes was tripping. We now just skip the validation if a badge already has a promo code.

Author: kitsuta

uber/validations/attendee.py

@@ -265,7 +265,7 @@ def out_of_badge_type(form, field):
 
 @PreregOtherInfo.new_or_changed('promo_code_code')
 def can_use_promo_code(form, field):
-    if not field.data:
+    if not field.data or form.model.promo_code:
         return
 
     if form.model.badge_type == c.PSEUDO_DEALER_BADGE or getattr(form.model, 'is_dealer', False):