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 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.

This was overridden in the fields where this resolver was used. So
this change is purely cosmetic.

- All definitions have been replaced by classes: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- Loaders have been replaced by resolvers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

By specifying a presenter for the object type, we can keep the logic
out of `GitlabSchema`.

The presenter gets initialized using the object being presented, and
the context (including the `current_user`).