ApiParse: Add 'useskin' param for general OutputPage/Skin support

ApiParse: Add 'useskin' param for general OutputPage/Skin support

Required for Idc41934eb89.

* If 'useskin' is set, run ParserOutput through OutputPage (with proper
  Skin set up). Specifically call addParserOutputMetadata().

  Then use OutputPage isntead of ParserOutput to retrieve that subset
  of meta data. Such as modules, lang links and config vars.

* Deprecate 'effectivelanglinks' in favour of 'useskin'.

* Simplify 'headhtml' support and re-use this new code.

Change-Id: Iaa85ac49f6e0cbdf7f1bb0f50a8f7730d119f0a2