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
```

cmd/mattermost/commands/server.go

@@ -73,7 +73,7 @@ func runServer(configStore *config.Store, interruptChan chan os.Signal) error {
 	}
 	server, err := app.NewServer(options...)
 	if err != nil {
-		mlog.Critical(err.Error())
+		mlog.Error(err.Error())
 		return err
 	}
 	defer server.Shutdown()
@@ -86,20 +86,24 @@ func runServer(configStore *config.Store, interruptChan chan os.Signal) error {
 		if x := recover(); x != nil {
 			var buf bytes.Buffer
 			pprof.Lookup("goroutine").WriteTo(&buf, 2)
-			mlog.Critical("A panic occurred",
+			mlog.Error("A panic occurred",
 				mlog.Any("error", x),
 				mlog.String("stack", buf.String()))
 			panic(x)
 		}
 	}()
 
-	api := api4.Init(server)
+	api, err := api4.Init(server)
+	if err != nil {
+		mlog.Error(err.Error())
+		return err
+	}
 	wsapi.Init(server)
 	web.New(server)
 
 	err = server.Start()
 	if err != nil {
-		mlog.Critical(err.Error())
+		mlog.Error(err.Error())
 		return err
 	}
 

cmd/mattermost/commands/test.go

@@ -58,7 +58,10 @@ func webClientTestsCmdF(command *cobra.Command, args []string) error {
 		return serverErr
 	}
 
-	api4.Init(a.Srv())
+	_, err = api4.Init(a.Srv())
+	if err != nil {
+		return err
+	}
 	wsapi.Init(a.Srv())
 	a.UpdateConfig(setupClientTests)
 	runWebClientTests()
@@ -79,7 +82,10 @@ func serverForWebClientTestsCmdF(command *cobra.Command, args []string) error {
 		return serverErr
 	}
 
-	api4.Init(a.Srv())
+	_, err = api4.Init(a.Srv())
+	if err != nil {
+		return err
+	}
 	wsapi.Init(a.Srv())
 	a.UpdateConfig(setupClientTests)