The TransformHandler class should be split into two handlers:

• WikitextTransformHandler takes wikitext and returns HTML or linter info.
• HtmlTransformHandler takes HTML and returns wikitext.

Both handler classes shold support PageBundles, as input or output respectively.

We may need a third handler class for HTML-to-HTML (pb2pb) transformations, but it is not clear yet whetehr we need to expose this functionality via REST.