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/api.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 
 	"github.com/gorilla/mux"
+	graphql "github.com/graph-gophers/graphql-go"
 	_ "github.com/mattermost/go-i18n/i18n"
 
 	"github.com/mattermost/mattermost-server/v6/app"
@@ -15,8 +16,9 @@ import (
 )
 
 type Routes struct {
-	Root    *mux.Router // ''
-	APIRoot *mux.Router // 'api/v4'
+	Root     *mux.Router // ''
+	APIRoot  *mux.Router // 'api/v4'
+	APIRoot5 *mux.Router // 'api/v5'
 
 	Users          *mux.Router // 'api/v4/users'
 	User           *mux.Router // 'api/v4/users/{user_id:[A-Za-z0-9]+}'
@@ -136,10 +138,11 @@ type Routes struct {
 
 type API struct {
 	srv        *app.Server
+	schema     *graphql.Schema
 	BaseRoutes *Routes
 }
 
-func Init(srv *app.Server) *API {
+func Init(srv *app.Server) (*API, error) {
 	api := &API{
 		srv:        srv,
 		BaseRoutes: &Routes{},
@@ -147,6 +150,7 @@ func Init(srv *app.Server) *API {
 
 	api.BaseRoutes.Root = srv.Router
 	api.BaseRoutes.APIRoot = srv.Router.PathPrefix(model.APIURLSuffix).Subrouter()
+	api.BaseRoutes.APIRoot5 = srv.Router.PathPrefix(model.APIURLSuffixV5).Subrouter()
 
 	api.BaseRoutes.Users = api.BaseRoutes.APIRoot.PathPrefix("/users").Subrouter()
 	api.BaseRoutes.User = api.BaseRoutes.APIRoot.PathPrefix("/users/{user_id:[A-Za-z0-9]+}").Subrouter()
@@ -292,12 +296,15 @@ func Init(srv *app.Server) *API {
 	api.InitSharedChannels()
 	api.InitPermissions()
 	api.InitExport()
+	if err := api.InitGraphQL(); err != nil {
+		return nil, err
+	}
 
 	srv.Router.Handle("/api/v4/{anything:.*}", http.HandlerFunc(api.Handle404))
 
 	InitLocal(srv)
 
-	return api
+	return api, nil
 }
 
 func InitLocal(srv *app.Server) *API {