CK Favour of the Dark Gods needs handling properly

[ThePants999]

The Chaos Knights upgrade Favour of the Dark Gods is (sensibly) represented as a multi-characteristic profile on a particular model. Under 5bb8462 I've added a simplistic initial handling which gets that model separated out with the ability listed in its tooltip, but we need more than that - we need it separated into the Standard and Favoured abilities, and the descriptive text for both added to the datacard. (And we need a unit test.)

Since there's no back-reference from models to the parent unit, I think this needs a fairly substantial change where Model.abilities becomes an array of {name, description} tuples instead of a simple array of names, and then something needs to happen on the unit to pull model-specific abilities into the unit.