Use the MWDebug class to display debug log back in the request.

Use the MWDebug class to display debug log back in the request.

- This removes the duplication of MWDebug::$debug and
  OutputPage::$mDebugtext, so there's no need to store two times
  the same text
- Removed OutputPage::debug() since it's no longer used
- Had to keep OutputPage::$mDebugtext because it's still
  referenced by SemanticPageMaker extension
- Moved Skin::formatDebugHTML() to MWDebug::GetHTMLDebugLog()
  and replaced the call in Skin::generateDebugHTML() to use it
- Also check $logonly before sending entries to the debug
  toolbar in wfDebug(), for consistency
- Changed MWDebug::getDebugHTML() to also return the debug log
  in an HTML comment if $wgDebugComments is set to true and
  changed the location of this call to BaseTemplate::printTrail()
  so that its result is the latest possible. This also includes
  the debug toolbar.
- Removed MockOutputPage and related test cases since they are
  no longer accurate

Change-Id: Ie0f389f8566457b1c938c627ed930040741ac9d9