mw.ViewPageTarget: Use .mw-body-content for content styling

* font-size and line-height differed between read mode and
  edit mode. This makes them match by using the same css class
  as MediaWiki core uses for the body content.
  Depends on I66c56c57 in MediaWiki core.

* Document padding / margin hack on surface, and sync the values.
  In edit mode, the text was being nudged to the right by a few
  pixels.

* Bump the wfUseMw call as this is a symptom of how VisualEditor
  now depends on MediaWiki 1.23.

Bug: 64599
Change-Id: I5818dce626545c4cbc8bd84fd551dc08baedbe32