Admin UI: denote settings that may be overridden in settings.php

Related to #2805

This is a feature implemented in D8 by https://www.drupal.org/project/coi and https://www.drupal.org/project/config_override_core_fields

Config Override Inspector provides indicators to administrators where form fields represent configuration that is overridden.

This module is especially useful for developers working on sites where there are configuration overrides in place depending on the environment. It prevents confusion that arise when changing a form setting, but appears to not take effect.