This extracts two helper classes from PageHTMLHandler:
* PageContentHelper for accessing page content. This replaces the
LatestRevisionContentHandler mase class.
* ParsoidHtmlHelper for generating HTML from wikitext using parsoid.
The idea is to decouple the functionality from the REST handlers, so we
can easily mix and match functionality to create a handler for the
new per-revision HTML endpoint.
Bug: T267981
Bug: T267982
Change-Id: I3226833d12e51c959712d642b0195de1fe1ef979
