Univention Corporate Server 4.4 erratum 96 (bugfix)

This update addresses the following issues:
* Docker image verification was removed from the App Center code. Instead,
  we rely on the functions of Docker itself.
* The App Center waits for a dpkg lock before attempting to install database
  software packages. If starting the database service fails anyway, we now
  send more verbose information.
* The package univention-appcenter is now built with dh_python2.
* The App Center is now prepared to show app suggestions based on already
  installed apps.
* A visual flickering in the App Center overview has been removed.