Feature Flag schema at CartoDB

This respects the documented usage, although loading feature flags from the local database.
Next step is Central sync.

@rambo, @Kartones , @rilla could you take a look? I'll do the sync logic meanwhile.
Despite now not usually having local SSL, it is nice to have also: 
```ssl_allowed :create, :update, :destroy unless Rails.env.production? || Rails.env.staging?```

Because for example when building multiuser I had SSL activated even in dev (and hit some errors), so better not to restrict, just make optional.