Adds more friendly docs for GraphQl implementation

- Remove `FeatureConstrainer` call wrapping api endpoint
- Remove `Feature.enabled?(:graphql)` conditionals in back and frontend
- Modify graphql test to be graphql flag agnostic
- Remove api routing spec
- Remove frontend feature flag via `gon`

Previously, we used brackets to denote the tier badges,
but this made Kramdown, the docs site Markdown renderer,
show many warnings when building the site. This is now
fixed by using parentheses instead of square brackets.

This was caused by [PREMIUM] looking like a link to
Kramdown, which couldn't find a URL there.

See:
- https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/484
- https://gitlab.com/gitlab-org/gitlab-ce/issues/63800

We can query namespaces, and nested projects.

Projects now exposes statistics

Enabling GraphQL batch requests allows for multiple queries
to be sent in 1 request reducing the amount of requests
we send to the server.

Responses come come back in the same order as the queries were
provided.

Replace all '9koXpg98eAheJpvBs5tK' occurrences with
'<your_access_token>' in API docs.

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- All definitions have been replaced by classes: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- Loaders have been replaced by resolvers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

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`).