Currently, the Codex implementations of each HTMLFormField subclass have a getInputHtml() method that checks whether the parent form is a CodexHTMLForm, and generates either the Codex version or the plain HTML version of the field based on that.

Instead, we should structure this more like how OOUIHTMLForm works, where there's a separate method in HTMLFormField (getInputOOUI()) that OOUIHTMLForm calls instead of getInputHtml().

• Add getInputCodex() method (or whatever we call it) to HTMLFormField, call it in CodexHTMLForm https://gerrit.wikimedia.org/r/c/mediawiki/core/+/1008569
• Migrate existing Codex field implementations to the new method
  • HTMLButtonField
  • HTMLCheckField
  • HTMLTextField

This new method should be properly documented and should be covered by PHPUnit tests.