Used Actor object for actor in Activity

ref https://linear.app/tryghost/issue/MOM-126

Similar to using the Article object for object in Activity, this allows us to
more easily pull out all of the extra data for Actors. At the moment its the
full JSONLD representation, but we can slim that down in future.

ghost/ghost/src/core/activitypub/activity.entity.ts

@@ -1,4 +1,5 @@
 import {Entity} from '../../common/entity.base';
+import {Actor} from './actor.entity';
 import {Article} from './article.object';
 import {ActivityPub} from './types';
 import {URI} from './uri.object';
@@ -6,7 +7,7 @@ import {URI} from './uri.object';
 type ActivityData = {
     activity: URI | null;
     type: ActivityPub.ActivityType;
-    actor: URI;
+    actor: Actor | URI;
     object: {
         id: URI;
         type: string;
@@ -53,7 +54,21 @@ export class Activity extends Entity<ActivityData> {
         return this.attr.object;
     }
 
+    getActor(url: URL) {
+        if (this.attr.actor instanceof Actor) {
+            return this.attr.actor.getJSONLD(url);
+        }
+        return {
+            type: 'Person',
+            id: this.attr.actor.getValue(url),
+            preferredUsername: 'index'
+        };
+    }
+
     get actorId() {
+        if (this.attr.actor instanceof Actor) {
+            return this.attr.actor.actorId;
+        }
         return this.attr.actor;
     }
 
@@ -70,15 +85,12 @@ export class Activity extends Entity<ActivityData> {
 
     getJSONLD(url: URL): ActivityPub.Activity {
         const object = this.getObject(url);
+        const actor = this.getActor(url);
         return {
             '@context': 'https://www.w3.org/ns/activitystreams',
             id: this.activityId?.getValue(url) || null,
             type: this.attr.type,
-            actor: {
-                type: 'Person',
-                id: this.actorId.getValue(url),
-                username: `@index@${this.actorId.hostname}`
-            },
+            actor: actor,
             object: {
                 ...object,
                 id: this.objectId.getValue(url)

ghost/ghost/src/core/activitypub/actor.entity.ts

@@ -156,7 +156,7 @@ export class Actor extends Entity<ActorData> {
             activity: new URI(`activity/${new ObjectID().toHexString()}`),
             to: this.followersCollectionId,
             type: 'Create',
-            actor: this.actorId,
+            actor: this,
             object: article
         });
         this.doActivity(activity);