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/controllers/activitypub/outboxes_controller_spec.rb

@@ -0,0 +1,19 @@
+require 'rails_helper'
+
+RSpec.describe ActivityPub::OutboxesController, type: :controller do
+  let!(:account) { Fabricate(:account) }
+
+  before do
+    Fabricate(:status, account: account)
+  end
+
+  describe 'GET #show' do
+    before do
+      get :show, params: { account_username: account.username }
+    end
+
+    it 'returns http success' do
+      expect(response).to have_http_status(:success)
+    end
+  end
+end