Univention Corporate Server 4.3 erratum 610 (extended maintenance)

This update addresses the following issues:
* Use-after-free when creating index updates in IndexedDB (CVE-2019-11757)
* Stack buffer overflow in HKDF output (CVE-2019-11759)
* Stack buffer overflow in WebRTC networking (CVE-2019-11760)
* Unintended access to a privileged JSONView object (CVE-2019-11761)
* document.domain-based origin isolation has same-origin-property violation
  (CVE-2019-11762)
* Incorrect HTML parsing results in XSS bypass technique (CVE-2019-11763)
* Memory safety bugs fixed in Firefox 70 and Firefox ESR 68.2
  (CVE-2019-11764)
* Heap-based buffer over-read via crafted XML input (CVE-2019-15903)