Add Sequence Subscribe Option

[n7studios]

Summary

Adds options to each WPForms Form to add the subscriber to a sequence when a form is submitted.

Testing

• testCreateFormToConvertKitSequenceMapping: Test that saving a WPFormsForm to ConvertKit Sequence Mapping works.

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)