Using abstracted checkout completion function

[dparker1005]

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Changes proposed in this Pull Request:

For the past year or so, we have used an abstracted "checkout completion" function to finish processing checkouts using the Stripe Checkout gateway. Now that this approach has been tested, this PR updates the PMPro checkout preheader code to use that abstracted (and renamed) function to use instead of having all the logic inline. The goal of this is to make the checkout flow more manageable to make a refactor easier down the road.

This also brings a couple of documentation updates and "hour" membership fixes from the inline preheader completion flow to the abstracted function.

There is also a bug where the $order->discount_code property is sometimes set to the discount code string instead of the discount code object. This could break some emails and is fixed in this PR.

Finally, the abstracted checkout completion function loses the logic to save billing address data in user meta, which is expected and the direction that we plan on moving.

How to test the changes in this Pull Request:

Checkouts have been tested with the free gateway, on-site Stripe, Stripe Checkout, and PayPal Express. For the paid tests, a discount code was used with all of them.

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Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

[ideadude]

Just some comments/questions. Great stuff. I'm doing a few more tests and will let you know how they go.