Prefix level and discount code parameters at checkout

[dparker1005]

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Changes proposed in this Pull Request:

Prefixing the level, discount_code, and other_discount_code parameters at checkout/confirmation. There is still code to support the non-prefixed parameters as a fallback.

Other changes include:

• Adding caching to the pmpro_getLevelAtCheckout() function
• Adding a new filter pmpro_default_discount_code for cases where you want to dynamically add a discount code at checkout (ex Sitewide Sales)
• Including the discount code used in the level object returned from pmpro_getLevelAtCheckout() if a code was used

With these changes, we should no longer have to reply on the $pmpro_level or $discount_code globals either. We should instead always call the pmpro_getLevelAtCheckout() function whenever that information is needed, which should be fairly quick now that it is cached.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

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[ideadude]

Fix the caching then merge. We will need to test a bunch, but this is all good otherwise.

[ideadude]

Have to fix this to avoid warnings when the isset check is made.

Either a better check and setting up the array of arrays on demand. Or concat the level_id . ',' . $discount_code and hash that and use the hash for the cache key. And/or use the WP get/set cache functions.

[ideadude]

Fix cache code here too.