[13.x] Discount improvements

[driesvints]

This PR implements some improvements and new API's for discount handling.

First of all, a new PromotionCode class was added that wraps a Stripe\PromotionCode object. This object represents a promotion code applied to either a customer or subscription.

Further, the Discount class now also features a promotionCode method to easily retrieve the promotion code should one have been used to apply a discount. Besides that, the class got dedicated methods to transform the start and end timestamps of a discount into Carbon\Carbon objects.

Other than that it's now easy to retrieve a PromotionCode by its code:

// Find the promotion code with 25OFF...
$promotionCode = $billable->findPromotionCode('25OFF');

// Or find only active promotion codes...
$promotionCode = $billable->findActivePromotionCode('25OFF');

It's now also easy to retrieve the applied discount on a customer or subscription:

$discount = $billable->discount();

$discount = $subscription->discount();

And it's easy now to apply a new coupon or promotion code on either a customer or subscription. These need the ID of their respective object:

$billable->applyCoupon('coupon_id');
$billable->applyPromotionCode('promotion_code_id');

$subscription->applyCoupon('coupon_id');
$subscription->applyPromotionCode('promotion_code_id');

I was thinking to add also some validation rules like coupon, promotion_code or coupon_or_promotion_code but decided to not extend the scope of this PR any further.

Closes #852