Add ActivityPub inbox (#4216)

* Add ActivityPub inbox

* Handle ActivityPub deletes

* Handle ActivityPub creates

* Handle ActivityPub announces

* Stubs for handling all activities that need to be handled

* Add ActivityPub actor resolving

* Handle conversation URI passing in ActivityPub

* Handle content language in ActivityPub

* Send accept header when fetching actor, handle JSON parse errors

* Test for ActivityPub::FetchRemoteAccountService

* Handle public key and icon/image when embedded/as array/as resolvable URI

* Implement ActivityPub::FetchRemoteStatusService

* Add stubs for more interactions

* Undo activities implemented

* Handle out of order activities

* Hook up ActivityPub to ResolveRemoteAccountService, handle
Update Account activities

* Add fragment IDs to all transient activity serializers

* Add tests and fixes

* Add stubs for missing tests

* Add more tests

* Add more tests

spec/lib/activitypub/activity/update_spec.rb

@@ -0,0 +1,41 @@
+require 'rails_helper'
+
+RSpec.describe ActivityPub::Activity::Update do
+  let!(:sender) { Fabricate(:account) }
+  
+  before do
+    sender.update!(uri: ActivityPub::TagManager.instance.uri_for(sender))
+  end
+
+  let(:modified_sender) do 
+    sender.dup.tap do |modified_sender|
+      modified_sender.display_name = 'Totally modified now'
+    end
+  end
+
+  let(:actor_json) do
+    ActiveModelSerializers::SerializableResource.new(modified_sender, serializer: ActivityPub::ActorSerializer, key_transform: :camel_lower).as_json
+  end
+
+  let(:json) do
+    {
+      '@context': 'https://www.w3.org/ns/activitystreams',
+      id: 'foo',
+      type: 'Update',
+      actor: ActivityPub::TagManager.instance.uri_for(sender),
+      object: actor_json,
+    }.with_indifferent_access
+  end
+
+  describe '#perform' do
+    subject { described_class.new(json, sender) }
+
+    before do
+      subject.perform
+    end
+
+    it 'updates profile' do
+      expect(sender.reload.display_name).to eq 'Totally modified now'
+    end
+  end
+end