feat(flag): experiment feature flag

Sometimes you just wanna turn this thing off from config because third
party services be third party servicin.

www/experiments.php

@@ -245,7 +245,7 @@ function observationHTML($parts)
                                 }
 
                                 // experiments are enabled for the following criteria
-                                $experimentEnabled = $experiments_paid || ( in_array($expNum, $allowedFreeExperimentIds));
+                                $experimentEnabled = !Util::getSetting('experiments_disabled') && ($experiments_paid || ( in_array($expNum, $allowedFreeExperimentIds)));
                                 // exception allowed for tests on the metric times
                                 if (strpos($test['testinfo']['url'], 'webpagetest.org/themetrictimes')) {
                                     $experimentEnabled = true;