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
```

vendor/github.com/graph-gophers/graphql-go/errors/errors.go

@@ -0,0 +1,59 @@
+package errors
+
+import (
+	"fmt"
+)
+
+type QueryError struct {
+	Err           error                  `json:"-"` // Err holds underlying if available
+	Message       string                 `json:"message"`
+	Locations     []Location             `json:"locations,omitempty"`
+	Path          []interface{}          `json:"path,omitempty"`
+	Rule          string                 `json:"-"`
+	ResolverError error                  `json:"-"`
+	Extensions    map[string]interface{} `json:"extensions,omitempty"`
+}
+
+type Location struct {
+	Line   int `json:"line"`
+	Column int `json:"column"`
+}
+
+func (a Location) Before(b Location) bool {
+	return a.Line < b.Line || (a.Line == b.Line && a.Column < b.Column)
+}
+
+func Errorf(format string, a ...interface{}) *QueryError {
+	// similar to fmt.Errorf, Errorf will wrap the last argument if it is an instance of error
+	var err error
+	if n := len(a); n > 0 {
+		if v, ok := a[n-1].(error); ok {
+			err = v
+		}
+	}
+
+	return &QueryError{
+		Err:     err,
+		Message: fmt.Sprintf(format, a...),
+	}
+}
+
+func (err *QueryError) Error() string {
+	if err == nil {
+		return "<nil>"
+	}
+	str := fmt.Sprintf("graphql: %s", err.Message)
+	for _, loc := range err.Locations {
+		str += fmt.Sprintf(" (line %d, column %d)", loc.Line, loc.Column)
+	}
+	return str
+}
+
+func (err *QueryError) Unwrap() error {
+	if err == nil {
+		return nil
+	}
+	return err.Err
+}
+
+var _ error = &QueryError{}

vendor/github.com/graph-gophers/graphql-go/errors/panic_handler.go

@@ -0,0 +1,18 @@
+package errors
+
+import (
+	"context"
+)
+
+// PanicHandler is the interface used to create custom panic errors that occur during query execution
+type PanicHandler interface {
+	MakePanicError(ctx context.Context, value interface{}) *QueryError
+}
+
+// DefaultPanicHandler is the default PanicHandler
+type DefaultPanicHandler struct{}
+
+// MakePanicError creates a new QueryError from a panic that occurred during execution
+func (h *DefaultPanicHandler) MakePanicError(ctx context.Context, value interface{}) *QueryError {
+	return Errorf("panic occurred: %v", value)
+}