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
```

shared/mlog/graphql.go

@@ -0,0 +1,23 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package mlog
+
+import (
+	"context"
+)
+
+// GraphQLLogger is used to log panics that occur during query execution.
+type GraphQLLogger struct {
+	logger *Logger
+}
+
+func NewGraphQLLogger(logger *Logger) *GraphQLLogger {
+	return &GraphQLLogger{logger: logger}
+}
+
+// LogPanic satisfies the graphql/log.Logger interface.
+// It converts the panic into an error.
+func (l *GraphQLLogger) LogPanic(_ context.Context, value interface{}) {
+	l.logger.Error("Error while executing GraphQL query", Any("error", value))
+}