"Unpublished references" dialog is not serializable

The refactoring in CMS-10690 made the "unpublished references" dialog not serializable anymore. This causes huge stacktraces when that dialog is opened while Wicket is running in development mode. It's also a code smell, since Wicket assumes that its state is serializable.

Steps to reproduce:

1. Start the CMS in Wicket development mode

mvn -Pcargo.run -Dcargo.jvm.args='-Dwicket.configuration=development'

1. Create a new document. Don't publish it.
2. Create another document with a link to the first one (i.e. an internal link in a rich text field).
3. Publish the second document. The "Unpublished references" dialog will pop up.

Expected: no errors logged.

Actual: a huge stack trace is logged (attached it since making it part of the description make the issue longer than 32767 characters, which JIRA does not accept).