Univention Corporate Server 4.1 erratum 218 (bugfix)

This update addresses the following issue(s):
* Access to the object class "univentionApp" is now more restrictive.
* The AppCenter now displays a warning if a conflict between the systems
  network settings and the docker bridge network has been detected.
* When searching for Apps, the Vendor (and Maintainer) of the App is
  considered.
* Apps may now require a specific version to be already installed, before an
  upgrade is possible.
* When opening an App in the Gallery that is not yet installed, a
  notification is sent to Univention.
* When (un)installing an App, a notification is always sent to Univention. If
  the App does not say otherwise, this notification is anonymized.
* Minor code changes make other projects easier that rely on the App Center.
* When a non Docker version of an App was installed, the upgrade to a Docker
  version is prohibited unless the App says it is safe to upgrade.