Add Abandoned Cart Functionality

[n7studios]

Summary

This PR adds abandoned cart functionality, where visitors who add items to their cart, enter their email address at checkout and then do not complete checkout within a given time.

If enabled, and the cart is deemed abandoned:

• a subscriber is created in Kit, and
• they are subscribed to the specified tag.

This tag is removed if the visitor subsequently completes checkout.

Testing

Tests are added to this PR for ease of review.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)