Cherry-pick HtmlFormatter into production

Contains commits:
* Ic276e1604c5718e8568e120ddfb9a8fc13a682fc "Move HtmlFormatter from MobileFrontend"
* I5c6394f811f9cd14dc549d005b4583d7575e85aa "libxml_disable_entity_loader() just in case..."
* I69623d565826aacaa884c1c0a3ffec46ca1fb465 "Followup to missing call in Ic276e1604c5718e8568e120ddfb9a8fc13a682fc"

Change-Id: I4e3c5d9537b1ecffed0f62736784712187476bad
Merge "Cherry-pick HtmlFormatter into production" into wmf/1.22wmf17