[Feat] Coupon system

[rafaelsideguide]

WIP. Idea for solving #57

[nickscamara]

• Change the order in bill to check coupon before subscription

• Make sure to decrease the coupon credits when billed

• On the check credits, make sure to check for the sum of the coupons

• Make sure people with no subscriptions can use coupons and people with subscriptions as well.

[rafaelsideguide]

X = Number of free credits

Test Cases:

• User with No Subscription and No Coupons
  Expected Result: User should have access to X free credits.

• User with No Subscription and One 10-Credit Coupon
  Expected Result: User should have a total of X + 10 credits available; after use, the coupon balance should be zero.

• User with No Subscription and Five 10-Credit Coupons (Using 30 Credits)
  Expected Result: User should utilize 30 credits from the coupons; three coupons should be fully used (zero balance) and two coupons should remain with 10 credits each.

• User with No Subscription and Five 10-Credit Coupons (Using X + 50 Credits)
  Expected Result: User should utilize 150 credits; all five coupons should have zero balance at the end.

• User with Subscription and One 10-Credit Coupon (Using 20 Credits)
  Expected Result: User should use 10 credits from the coupon (leaving it with zero balance) and 10 credits from the subscription.

• User with Subscription and Five 10-Credit Coupons (Using 30 Credits)
  Expected Result: User should use 30 credits from the coupons; three coupons should be depleted (zero balance) and two should have 10 credits each remaining.

• User with Subscription and Five 10-Credit Coupons (Using 70 Credits)[
  Expected Result: User should use 50 credits from the coupons (leaving them with zero balance) and 20 credits from the subscription.

[nickscamara]

Lgtm! Great job Rafa!