Univention Corporate Server 4.4 erratum 455 (bugfix)

This update addresses the following issues:
* In some cases, an unsuccessful app installation was still acknowledged
  as successful. This fix ensures that the appcenter receives a correct 
  status code for those cases.
* During app installation there was a mix of popup dialogs and dedicated
  pages. These have been unified into an installation wizard.
* Errors in a cached response in the UMC module could lead to tracebacks. 
  This has been fixed.
* The warning message indicating a potentially long installation time of
  container based apps has been removed.
* The error tracking for failed docker pull requests has been improved.