Importer: MailPoet

[n7studios]

Summary

Adds an option in Settings > Kit > Tools to replace MailPoet forms (added by the MailPoet Plugin) with Kit forms:

Based on the form mappings selected, this searches all published WordPress Posts, Pages and Custom Posts for any MailPoet shortcodes and blocks, replacing them with the Kit form shortcodes and blocks.

Testing

• PluginSettingsToolsImporterMailPoetCest: Added end to end tests for migrating shortcodes and blocks
• ImporterTest: Added integration tests for MailPoet

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)