Make Release data available through GraphQL API

Problem to solve

Make Release data available through GitLab's GraphQL API in order to align with the GraphQL vision:

We want the GraphQL API to be the primary means of interacting programmatically with GitLab. To achieve this, it needs full coverage - anything possible in the REST API should also be possible in the GraphQL API.

Proposal

1. Add basic Release data to our GraphQL endpoint
2. Follow-up MR to add some of the more complex data (mostly just commit data) to the endpoint
   • It's possible we may not need this follow-up
3. Follow-up MR to add a few extra properties to the existing Milestone endpoint that we currently use in Release pages.