GraphQL setup: Basic Project and Merge request endpoint

What does this MR do?

This is based on the work done in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13601.

It removes some logic from the types, uses existing logic in presenters.

Other TODO:

• Add 'full stack' tests for each query with all fields?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/34754

EE-Port in: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5969