Prevent settings drift

[vpetersson]

So let's say you are working on a local app and introduce a new setting to screenly.yml with mock-data.yml present. It is then very easy to lose a lot of time trying to figure out why the new setting that you just introduced isn't available.

What instead would make a lot of sense is to have a 'drift detection' feature where if you introduce a new setting in screenly.yml that isn't present in mock-data.yml, we would raise an error.