feat(ras-acc) add welcome email template for checkout registration

[chickenn00dle]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Closes https://app.asana.com/0/1207385073694403/1205250932343413/f

This PR is partnered with the following main plugin PR: Automattic/newspack-plugin#3150

This adds a new welcome email template for readers who register via checkout as part of RAS-ACC.

How to test the changes in this Pull Request:

See Automattic/newspack-plugin#3150 for testing instructions.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[dkoo]

Working well! Just left a couple of comments related to this feature needing to work for non-donation transactions as well.

[dkoo]

Probably a redundant check if not changing the condition on line 163

[chickenn00dle]

@dkoo, I noticed an issue with the checkout button modal checkout throwing a session expired error in some cases so added some small changes in e8cadf2. This should be good for another round of review.

[dkoo]

Seems to be working now!