EZP-20673: Database check shows eznode_assignment error after 5.1 upgrade

This PR fixes issue https://jira.ez.no/browse/EZP-20673

This adds update queries to ```dbupdate-5.0.0-to-5.1.0.sql``` upgrade script that were omitted from ```dbupdate-4.7.0-to-5.0.0.sql``` for MySQL and PostgreSQL. Oracle is fine as those were added there.

Related:
https://github.com/ezsystems/ezpublish-legacy/pull/502
https://github.com/ezsystems/ezoracle/pull/8