Univention Corporate Server 4.1 erratum 79 (bugfix)

This update addresses the following issues:
* The layout of the App Center gallery has been improved.
* On an app details page, it is now possible to navigate to the previous and
  next app in the search results.
* The app details page for small domains has been simplified.
* Apps now support RequiredAppsInDomain which can be used to state that a
  specific App has to be installed somewhere in the domain in order for the
  current App to work.
* Performance optimizations: Caching all apps in one Python pickle file for
  faster rereading.
* Performance optimizations: Vastly decreasing the cost of using Univention
  Config Registry in the App Center module by using a singleton object.
* Fixing developer scripts for using a local App Center.
* One function of the Docker Apps Joinscript helpers has been fixed when
  called from within the UMC module.
* Uninstalling the last app doesn't cause a "Forbidden" pop up anymore.
* Calling univention-app does not log the complete set of options, instead
  just the name of the action being called.
* univention-app upgrade now correctly upgrades all packages for non Docker
  Apps.
* Add attribute 'univentionAppID' to the LDAP equality search index.
* Translation format strings with multiple unnamed arguments have been
  adjusted.
* joinscript_run_in_container from the Docker Apps Joinscript helper
  functions now correctly passes its arguments to univention-app shell