Univention Corporate Server 4.4 erratum 27 (bugfix)

This update addresses the following issue(s):
* Rely on the App Center listener to create the App Attributes mapping
  rather than doing it in the UMC module
* Preparations for better browser cache-control have been implemented.
* Prevent high CPU usage in the "Users" module.
* Default containers for domain controller computer objects can now be
  configured.