Use a string to define the class used by Audited

This is possible since Audited 5.2.0, and will make it easier to start
using Zeitwerk because it won't reference a constant in an initializer.

config/initializers/audited.rb

@@ -1,4 +1,4 @@
 Audited.config do |config|
-  config.audit_class = ::Audit
+  config.audit_class = "::Audit"
   config.ignored_default_callbacks = [:touch]
 end