API debugging flag $wgDebugAPI to disable some security checks

Intentionally introduce a big security hole for debugging only.
Just like $wgShowDebug, $wgShowExceptionDetails, and $wgShowSQLErrors,
that introduce security vulnarabilities for debugging purposes,
this flag allows developers to remove some security checks in their
LocalSettings.php. This is a much safer approach than temporarily
changing production code to ignore certain conditions and later
forgetting to remove those changes before commiting the code.
Whenever enabled, a warning will allways be added to all output.

Change-Id: Ideb271dc1c9087f0843da03d024d70cc2cd776ae