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/system_test.go

@@ -752,7 +752,8 @@ func TestServerBusy503(t *testing.T) {
 
 func TestPushNotificationAck(t *testing.T) {
 	th := Setup(t).InitBasic()
-	api := Init(th.Server)
+	api, err := Init(th.Server)
+	require.NoError(t, err)
 	session, _ := th.App.GetSession(th.Client.AuthToken)
 	defer th.TearDown()