Univention Corporate Server 4.3 erratum 573 (security)

This update addresses the following issues:
* Sandbox escape through Firefox Sync (CVE-2019-9812)
* Memory safety bugs fixed (CVE-2019-11740)
* Same-origin policy violation with SVG filters and canvas to steal
  cross-origin images (CVE-2019-11742)
* Cross-origin access to unload event attributes (CVE-2019-11743)
* XSS by breaking out of title and textarea elements using innerHTML
  (CVE-2019-11744)
* Use-after-free while manipulating video (CVE-2019-11746)
* Use-after-free while extracting a key value in IndexedDB (CVE-2019-11752)