fix(modal-checkout): support utm params

[miguelpeixe]

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Changes proposed in this Pull Request:

Ensures UTM parameters are passed as form inputs to the checkout modal.

How to test the changes in this Pull Request:

Further details and instructions at Automattic/newspack-plugin#2665

Other information:

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• Have you written new tests for your changes, as applicable?
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[adekbadek]

In my testing, this is not necessary. The form fields are already added with Automattic/newspack-plugin#2665 and I can complete testing successfully following instructions in Automattic/newspack-plugin#2665 without the changes from this PR. What am I missing?

[miguelpeixe]

You're right! The existing strategy picks up the URL and merges them to the other params for generating the checkout URL for redirection.

I kept digging into what inconsistency I was trying to fix with this and found an issue with the strategy from Automattic/newspack-plugin#2665. Here's a PR to fix that: Automattic/newspack-plugin#2671