Orders: Support Opt In on Manual Orders

[n7studios]

Summary

Resolves this ticket, by adding functionality on the WooCommerce Orders screen in the WordPress Administration, to opt in the order's customer to Kit, subscribing them to the Plugin, Product(s) and/or Coupon defined Forms, Tags and/or Sequences. This is typically used when a creator wants to manually create a WooCommerce Order.

Once a customer is subscribed, their status is displayed, preventing the possibility of re-subscribing them:

Testing

• testOptInWithManualOrder: Test that the customer is subscribed to Kit with the Plugin and Product resources, based on the Order status, when manually creating an order in WooCommerce

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)