Compare the configured base DNs of domains that are visible as to be able to associate child domain name spaces with parent domain name spaces.

Summary: Domain validation: Consider child domains by comparing base DNs

Test Plan: No particular test plan in particular

Reviewers: Web Administration Panel Developers, machniak

Reviewed By: Web Administration Panel Developers, machniak

Subscribers: dhoffend, machniak, pokorra

Maniphest Tasks: T435

Differential Revision: https://git.kolab.org/D10