This adds Keyset pagination to GraphQL lists. PoC for that is
pipelines on merge requests and projects.
When paginating a list, the base-64 encoded id of the ordering
field (in most cases the primary key) can be passed in the `before` or
`after` GraphQL argument.
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- All definitions have been replaced by classes: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- Loaders have been replaced by resolvers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
