[graphql] Add metrics to hyperboards

[bitbeckers]

When working with gainforest, we learned some metrics would be valuable to attach to the hyperboards/collections:

It would be great if you can suggest us the shortest no. of queries to get the following data:

1. total no. of sales across the hyperboard.
2. total no. of buyers accross the hyperboard.
3. total amount of sales (units sold, and price per unit)

This would require updated to the graphql entitity definitions. It could be smart to also expose the Collection entities directly instead of only Hyperboards. These are probably multiple stories and this issue is the starting point.

[pheuberger]

Collections entities are exposed as per #214.

I can have a look after the PR is merged to optimize the number of queries to get these data points.

[pheuberger]

Waiting for feedback from @Jipperism on the PR above and approval to merge.