add orange header with warning if dev/testing

[mboehn]

first php code in a while. lets see how this goes.

In OverrideConfig.php:
$envWarning = "This is a test environment";

This is useful so users know they are in the wrong place, and for us to remember that we're not live.

[hultberg]

Please declare $envWarning with default value false or add an isset check here as it will trigger an notice error in php.