Univention Corporate Server 4.3 erratum 249 (bugfix)

This update addresses the following issue(s):
* When an app is opened in the App Center, the page now scrolls to the
  top and when the App detail page is closed, the previous scroll position
  is restored.
* Proxy settings in containers are updated before an app upgrade.
* Installation routines now ask for valid credentials before actually doing
  anything. This prevents possible errors when reverting the installation.
  status should the credentials be invalid.
* Never prompt in container commands during the app installation.