Add the ability to query descendant milestones via REST API

Problem

Getting a list of all milestones 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/milestones API so you could get a list of all group and subgroup milestones via /groups/:id/milestones?include_descendants=true. The GraphQL API already supports this.