Firefox Labs integration

[freshstrangemusic]

It would be great to be able to create Firefox Labs recipes locally and inject them into ExperimentManager.optInRecipes.

[freshstrangemusic]

This will likely require Firefox changes, so that we only clear labs recipes with source == "rs-loader". Then we can add these recipes to the set of optIns with source = "nimbus-devtools" and they will not be removed by RS syncs.