Form Subscribe: Add referrer support

[n7studios]

Summary

Specifies the URL a WPForms Form is embedded in as the referrer parameter, when configured to subscribe the email address to a Kit Form.

Testing

• testCreateFormToConvertKitFormMapping: Updated test to confirm the subscriber is subscribed to the Kit Form, and has the expected referrer URL.

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
• The code passes when running PHPStan
• 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)