LL-294 Added logic for activities and recommendations.

Added ActivityData to hold activity methods. Also added Activity as a
type into schema (based on Thing). Loading activities should be fully
functional and only view implementation and handling different types is
missing (also contained entities are not loaded yet). Loading
recommendations is still missing a decorator on the Service Model side,
thus loaded entities could not yet be added to VIE. Loading for both new
types is still disabled.