Directly Remove Credentials on 401

[n7studios]

Summary

Implements Kit/convertkit-wordpress#964 and Kit/convertkit-wordpress#966 from the main Kit Plugin:

• move token hook logic to functions.php,
• checks the result of any refresh() call, forcibly removing credentials from the Plugin's settings if the failure is a 401
• on initialization of a resource class, such as Integrate_ConvertKit_WPForms_Resource_Forms, removes the ConvertKit_Resource_V4 init() call and loads cached resources.

All calls made to refresh() from the Plugin are made by the user when editing a WPForms Form and configuring the Marketing > Kit section:

Testing

• testAccessTokenDeletedWhenInvalid: Confirm the the access token is deleted when a call to refresh() is made.
• testInvalidCredentials: Confirm that a notice is displayed in the WordPress administration UI when an invalid access token was used

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)

[noelherrick]

This line appears unnecessary.