Univention Corporate Server 4.1 erratum 373 (bugfix)

This update addresses the following issues:
* Limit the hostname for Docker containers to 14 characters.
* Univention gets informed about clicks on the "Buy" button.
* Univention also recieves the systems uuid with the tracking information.
* Improved performance by reducing file access in the backend and only
  sending relevant data to the frontend when opening the App Center gallery.