Convert OutputHandler functions to a class

Convert OutputHandler functions to a class

* Convert OutputHandler.php from global functions to a class.

  - wfOutputHandler → OutputHandler::handle
    (no alias, no usage outside core)
  - wfGzipHandler → OutputHandler::handleGzip
    (private, no usage outside class)
  - wfRequestExtension → OutputHandler::findUriExtension
    (private, no usage outside class)
  - wfMangleFlashPolicy → OutputHandler::mangleFlashPolicy
    (private, no usage outside class)
  - wfDoContentLength → OutputHandler::emitContentLength
    (private, no usage outside class)
  - wfHtmlValidationHandler → OutputHandler::validateAllHtml
    (private, no usage outside class)

* Add the class to autoload.php for exposure outside WebStart.
  Specifically, for use in ApiFormatPhpTest. This also removes the
  need to manually load the class because this code runs after
  Setup.php loads AutoLoader.php.

Bug: T189966
Change-Id: I27a41ec0ae0ee30aeb313a616323b967605c4055