Switch from Parsoid extension's styles to MediaWiki core's

Using the new mediawiki.skinning.content.parsoid module instead of the clunky
ext.parsoid.styles (which added an unhelpful dependency).

Change-Id: I87e93777ece75ebf035df29490a80daf9fa7c45a
Merge "Switch from Parsoid extension's styles to MediaWiki core's"