Broadcasts Importer: Refactor Functions

[n7studios]

Summary

Refactors the ConvertKit_Broadcasts_Importer class, making it possible to unit test the import_broadcast method.

In the future, we'll likely mock the API call to the wordpress/posts endpoint in the WPUnit tests, to return a variety of different broadcast content to test the parsing and various configurations of the import_broadcast method.

Testing

• BroadcastsImportTest: Added WPUnit tests against the import_broadcast method for the currently supported configurations.

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)