Univention Corporate Server 4.1 erratum 150 (bugfix)

This update addresses the following issue(s):
* The UMC login page does not warn about an insecure connection when
  logging in from the same machine the UMC runs on.
* Error dialogs concerning Piwik are now suppressed.
* Ensure that the first non-empty UMC category is shown at startup.
* Add a hook interface for other packages for extending the behavior
  of UMC.