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
```

model/feature_flags.go

@@ -69,6 +69,9 @@ type FeatureFlags struct {
 
 	// Enable Workspace optimization dashboard
 	WorkspaceOptimizationDashboard bool
+
+	// Enable GraphQL feature
+	GraphQL bool
 }
 
 func (f *FeatureFlags) SetDefaults() {
@@ -93,6 +96,7 @@ func (f *FeatureFlags) SetDefaults() {
 	f.BoardsDataRetention = false
 	f.NormalizeLdapDNs = false
 	f.WorkspaceOptimizationDashboard = false
+	f.GraphQL = false
 }
 
 func (f *FeatureFlags) Plugins() map[string]string {