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
```

api4/oauth_test.go

@@ -4,7 +4,6 @@
 package api4
 
 import (
-	"io/ioutil"
 	"net/http"
 	"testing"
 
@@ -608,13 +607,6 @@ func TestGetAuthorizedOAuthAppsForUser(t *testing.T) {
 	require.NoError(t, err)
 }
 
-func closeBody(r *http.Response) {
-	if r != nil && r.Body != nil {
-		ioutil.ReadAll(r.Body)
-		r.Body.Close()
-	}
-}
-
 func TestNilAuthorizeOAuthApp(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()