Add the ability to query descendant iterations via REST and GraphQL API

Problem

Getting a list of all iterations within a group hierarchy requires recursively querying each subgroup. This becomes a scalability problem when you have thousands of groups and subgroups.

Proposal

Add support for an include_descendants param to GET /groups/:id/iterations and GraphQL API so you could get a list of all group and subgroup iterations via GET /groups/:id/iterations?include_descendants=true