Respect last_unit_added when deciding whether to publish
This is a follow-up to commits for issue #5951. Previous commits made it so that mutating an erratum unit would set last_unit_added on all affected repos; however, the code for deciding whether to skip publish ignored this field and only looked at RepositoryContentUnits, so the commits didn't fix the issue.
Fix this by checking last_unit_added as well, so we cover both the "new RepositoryContentUnits created for the repo" and "unit shared between multiple repos was updated" cases. Erratum units are the only type known to fall into the latter case.