Emit JS CustomEvent when a user subscribes to a ConvertKit Form

[n7studios]

Summary

Inspired by this PR, emits a CustomEvent when a user subscribes to a ConvertKit Form.

This is handy to have so other plugins, can listen to, and we can use it in our current ongoing integration for custom event tracking.

Use document.addEventListener( 'convertkit_user_subscribed', function( event ) { console.log( event ); } ); to capture the event:

Testing

Existing tests pass.

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)

[n7studios]

Noticed that this was missing, and as a result would mean the result in the next .then block would be null.