[13.x] Multiple discounts on receipts

[driesvints]

This PR implements multiple discount support for receipts. With these changes, all discounts on a receipt are properly displayed with their name set up through Stripe as well as their percentage or amount off.

Here's an example of multiple discounts:

The general amount of is calculated first. Afterwards, individual discounts are applied per specific product they apply for.

Here's the equivalent Stripe receipt:

Some other changes in this PR:

• I've refactored the way extra parameters on the Stripe Invoice object are expanded. They are now only expanded when called upon and the data is cached internally on the object.
• A new Discount and Coupon object were introduced to wrap the related Stripe objects.
• Cleaned up some inconsistencies with collection class instantiation and is_null calls.

Closes #1145

[driesvints]

I decreased the line-height here slightly to account for the extra items on the invoice.