This adds a reusable way to expose permissions for a user to types in
GraphQL.

This allows the user to get a single MR nested in a GraphQL project
query.

Since we need the full path and the iid anyway, this makes more sense
than having a root query that needs the full path as well.

- All definitions have been replaced by classes:
  http://graphql-ruby.org/schema/class_based_api.html
- Authorization & Presentation have been refactored to work in the
  class based system
- Loaders have been replaced by resolvers
- Times are now coersed as ISO 8601