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/lib/activitypub/activity/update.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class ActivityPub::Activity::Update < ActivityPub::Activity
+  def perform
+    case @object['type']
+    when 'Person'
+      update_account
+    end
+  end
+
+  private
+
+  def update_account
+    return if @account.uri != object_uri
+    ActivityPub::ProcessAccountService.new.call(@account.username, @account.domain, @object)
+  end
+end