Configuration Override Warn

Implements the warning message proposed by #2408549: Display status message on configuration forms when there are overridden values from one module without needing any changes to configuration forms.

This module uses PHP reflection to modify a core class method from protected to public. Typically, it is not recommended to use reflection in production code/environments, but there's no way around it for this module and it should be a minimal impact to performance since it only affects pages that have configuration forms.