feat: serve Tombstone objects for soft deleted posts

re: #12551

src/activitypub/mocks.js

@@ -212,6 +212,18 @@ Mocks.actors.category = async (cid) => {
 
 Mocks.note = async (post) => {
 	const id = `${nconf.get('url')}/post/${post.pid}`;
+
+	// Return a tombstone for a deleted post
+	if (post.deleted === true) {
+		return Mocks.tombstone({
+			id,
+			formerType: 'Note',
+			attributedTo: `${nconf.get('url')}/uid/${post.user.uid}`,
+			context: `${nconf.get('url')}/topic/${post.topic.tid}`,
+			audience: `${nconf.get('url')}/category/${post.category.cid}`,
+		});
+	}
+
 	const published = new Date(parseInt(post.timestamp, 10)).toISOString();
 
 	// todo: post visibility
@@ -341,3 +353,9 @@ Mocks.note = async (post) => {
 
 	return object;
 };
+
+Mocks.tombstone = async properties => ({
+	'@context': 'https://www.w3.org/ns/activitystreams',
+	type: 'Tombstone',
+	...properties,
+});