Fix EZP-23607: display actual published version in versionview

JIRA: https://jira.ez.no/browse/EZP-23607

When viewing an old/archived version through versionview, object info shows incorrect "Published Version" information.

This resolves it by adding an new "published_version" function attr. and updating relevant template.
Feedback welcome
Please don't return `false` as it makes the method return type inconsistent. Prefer `null` instead.