Move WordPress Libraries Methods to Trait

[n7studios]

Summary

To facilitate using the ConvertKit_API_Traits trait directly in the Kit WordPress Libraries, without any changes, this PR standardises the trait by:

• Adding the get_body_html method, which uses DOMDocument vs. string search/replace
• Removes the unused and deprecated strip_html_head_body_tags method
• Fixes a bug in convert_relative_to_absolute_urls that would stop after the first change to a node in the NodeList

Testing

• ConvertKitMethodsTest: Test the convert_relative_to_absolute_urls and get_body_html methods work correctly with unit tests

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
• The code passes when running PHPStan
• 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)