Univention Corporate Server 4.1 erratum 146 (bugfix)

This update addresses the following issue(s):
* Improved the image gallery in the details page of an app. The
  handling of YouTube videos has been improved and enlarging
  thumbnails is smoother.
* A helptext still mentioned the Free-for-personal-Use Edition. This
  has been replaced with the UCS Core Edition.
* The overview of software changes has been enhanced.
* A margin has been added to the footer buttons of the installation
  error page.
* Join scripts and unjoin scripts are now correctly removed while
  installing and uninstalling an App.
* Minor code changes make other projects easier that rely on the App
  Center.
* univention-app shell now correctly exits with the exit code of the
  called command.
* univention-app installed unsigned packages.
* The "Package Management" module can be opened via hash again.
* Details of software packages can be shown again.
* The installation status of Docker Apps was determined incorrectly
  by certain functions of the App Center. This has been fixed.
* Joinscripts of Docker Apps are run via univention-run-join-scripts
  instead of being called directly.