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

app/services/activitypub/fetch_remote_status_service.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+class ActivityPub::FetchRemoteStatusService < BaseService
+  include JsonLdHelper
+
+  # Should be called when uri has already been checked for locality
+  def call(uri)
+    @json = fetch_resource(uri)
+
+    return unless supported_context? && expected_type?
+
+    attributed_to = first_of_value(@json['attributedTo'])
+    attributed_to = attributed_to['id'] if attributed_to.is_a?(Hash)
+
+    return unless trustworthy_attribution?(uri, attributed_to)
+
+    actor = ActivityPub::TagManager.instance.uri_to_resource(attributed_to, Account)
+    actor = ActivityPub::FetchRemoteAccountService.new.call(attributed_to) if actor.nil?
+
+    ActivityPub::Activity::Create.new({ 'object' => @json }, actor).perform
+  end
+
+  private
+
+  def trustworthy_attribution?(uri, attributed_to)
+    Addressable::URI.parse(uri).normalized_host.casecmp(Addressable::URI.parse(attributed_to).normalized_host).zero?
+  end
+
+  def supported_context?
+    super(@json)
+  end
+
+  def expected_type?
+    %w(Note Article).include? @json['type']
+  end
+end