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/custom_status.go

@@ -8,6 +8,8 @@ import (
 	"encoding/json"
 	"fmt"
 	"time"
+
+	"github.com/graph-gophers/graphql-go"
 )
 
 const (
@@ -61,6 +63,12 @@ func (cs *CustomStatus) AreDurationAndExpirationTimeValid() bool {
 	return false
 }
 
+// ExpiresAt_ returns the time in a type that has the marshal/unmarshal methods
+// attached to it.
+func (cs *CustomStatus) ExpiresAt_() graphql.Time {
+	return graphql.Time{Time: cs.ExpiresAt}
+}
+
 func RuneToHexadecimalString(r rune) string {
 	return fmt.Sprintf("%04x", r)
 }