ve.ce.Surface: Move insert HTML path to DM

Provided through new ve.dm.SurfaceFragment#insertHtml and
ve.dm.SurfaceFragment#insertDocument methods via
ve.dm.Document#newFromHtml, so it can be used by other pathways.

Co-Authored by: Ed Sanders <esanders@wikimedia.org>
Change-Id: I17191f6ffc18cc1f0d780ae48a53d39ceec35643