GraphQL (part 2): Model changes and resolvers (#19486)

This PR adds the necessary resolvers to support
the graphQL front-end. Some additional methods
need to be added to the model structs to support
this. Comments have been made to clarify their purpose.

There is a slight duplication of code in the api layer.
But eventually that's going to go away when we move
away from the REST API entirely.

The schema is in api4/schema.graphqls and gets compiled
into the binary as an asset.

The GraphiQL editor url is at GET /api/v5/graphql.

Everything is behind the feature flag MM_FEATUREFLAGS_GRAPHQL.

```release-note
NONE
```

go.mod

@@ -39,6 +39,7 @@ require (
 	github.com/gorilla/mux v1.8.0
 	github.com/gorilla/schema v1.2.0
 	github.com/gorilla/websocket v1.4.2
+	github.com/graph-gophers/graphql-go v1.3.0
 	github.com/h2non/go-is-svg v0.0.0-20160927212452-35e8c4b0612c
 	github.com/hako/durafmt v0.0.0-20210608085754-5c1018a4e16b
 	github.com/hashicorp/errwrap v1.1.0 // indirect