Discount code causes issues with checkouts

[MaximilianoRicoTabo]

• Hook into pmpro_include_payment_information_fields filter and set global $pmpro_requirebilling to true if child account seats have cost.

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Changes proposed in this Pull Request:

Resolves #17

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[MaximilianoRicoTabo]

Also with this fix I think we could remove warning regarding free levels since checkout will work.

Thoughts @dparker1005 @kimcoleman ?

[dparker1005]

#33 contains a similar fix, but does not fix the case where a free discount code is added to checkout via AJAX and CC fields disappear. This is likely an issue that needs to be addressed in core PMPro. Specifically in core, I think maybe we need to use the pmpro_checkout_level filter around here: https://github.com/strangerstudios/paid-memberships-pro/blob/fd6bf1c18315c71013da26d4ab8c0f24e6640591/services/applydiscountcode.php#L80-L87

I am keeping this PR open as a draft as this is a valid workaround for sites that run into this issue going forwards, but this solution will likely not be merged.