Page Previews should be able to consume HTML response generated by
MediaWiki. First we need to move out plain text crunching from
renderer.js and model.js. Mediawiki and Restbase gateways will have
to parse/htmlize plaintext into nice HTML by themselves.
Bug: T165018
Change-Id: I5d7e9f610bb809aa9fb035a4a9f96e9e8796c9d8
