Categories: Forms: Add None Option

[n7studios]

Summary

Adds a None option as requested here, matching how Post and Plugin level settings operate, to not display a Form should a Post be assigned to the given Category.

Testing

• testAddCategoryWithNoneFormSetting: Test that adding a Category with the Form setting = None results in no Form being displayed when viewing a Post.
• testEditCategoryWithNoneFormSetting: Test that editing a Category with the Form setting = None results in no Form being displayed when viewing a Post.

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)