We now keep track of the source of Firefox Labs opt-ins when registering them with the ExperimentManager. Currently all opt-ins are coming from Remote Settings, but this will pave the way for supporting force enrollment (via about:studies URLs or nimbus-devtools) and test enrollment (from nimbus-devtools). Additionally the ExperimentManager.getSingleOptInRecipe method has been removed because it was only used inside tests. Differential Revision: https://phabricator.services.mozilla.com/D297470