Federate pinned statuses over ActivityPub (#6610)

* Federate pinned statuses over ActivityPub

* Display pinned toots in web UI

Fix #6117

* Fix migration

* Fix tests

* Update outbox_serializer.rb

* Update remove_serializer.rb

* Update add_serializer.rb

* Update fetch_featured_collection_service.rb

spec/lib/activitypub/activity/add_spec.rb

@@ -0,0 +1,29 @@
+require 'rails_helper'
+
+RSpec.describe ActivityPub::Activity::Add do
+  let(:sender) { Fabricate(:account, featured_collection_url: 'https://example.com/featured') }
+  let(:status) { Fabricate(:status, account: sender) }
+
+  let(:json) do
+    {
+      '@context': 'https://www.w3.org/ns/activitystreams',
+      id: 'foo',
+      type: 'Add',
+      actor: ActivityPub::TagManager.instance.uri_for(sender),
+      object: ActivityPub::TagManager.instance.uri_for(status),
+      target: sender.featured_collection_url,
+    }.with_indifferent_access
+  end
+
+  describe '#perform' do
+    subject { described_class.new(json, sender) }
+
+    before do
+      subject.perform
+    end
+
+    it 'creates a pin' do
+      expect(sender.pinned?(status)).to be true
+    end
+  end
+end