Add Notice Recommending Official Add-On

[n7studios]

Summary

As requested here:

• Adds a notice in the WordPress Administration screens recommending to install and activate the official Gravity Forms ConvertKit Add-On:

• Updates the readme file to again recommend the official add-on.

Once the official add-on is installed and activated, it perform its own check to determine whether this Plugin is active, deactivating it if so. They retain our settings.

Testing

• NoticesCest:testPersistentNoticeDisplays: Test that the persistent notice displays when this Plugin is active.
• NoticesCest:testPersistentNoticeNotDisplayedWhenOfficialAddonActive: Test that the persistent notice does not display when the official add-on is activate.

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)