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/graphql_test.go

@@ -0,0 +1,34 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package api4
+
+import (
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestGraphQLPayload(t *testing.T) {
+	os.Setenv("MM_FEATUREFLAGS_GRAPHQL", "true")
+	defer os.Unsetenv("MM_FEATUREFLAGS_GRAPHQL")
+
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+
+	largeString := strings.Repeat("hello", 204800)
+
+	input := graphQLInput{
+		OperationName: "config",
+		Query:         largeString,
+	}
+
+	resp, err := th.MakeGraphQLRequest(&input)
+	require.NoError(t, err)
+	require.Len(t, resp.Errors, 1)
+	// The actual error isn't exposed. We compare the string
+	// to not confuse with other errors.
+	require.Contains(t, resp.Errors[0].Message, "request body too large")
+}