Add additional content and followable checks to home page artwork modules

Ideally the response would provide some sort of context for these cases so they don’t have to be individually checked for by the client:

So fields like has_additional_content and followable would be nice. Maybe is_followed_by_user too.

[broskoski]

@sarahscott for the related content, it might be useful to look at how we handle this on Force.

For instance, an individual module: https://github.com/artsy/force/blob/master/apps/home_2/queries/module.coffee

context is a GraphQLUnion type that is switched depending on the type of module returned. We use these contexts switches to get additional info on the module (e.g. Auction metadata, Related artist “based on”, etc).

For the Related Artists example: https://github.com/artsy/force/blob/master/apps/home_2/components/related_artists_context/query.coffee

Context definitions are here: https://github.com/artsy/metaphysics/blob/master/schema/home/context.js

[alloy]

@sarahscott @broskoski what are your thoughts on this nowadays?