[DAV] Fix amendment functionality
While amendments work in the AlekSIS frontend, it does not work reliably in CalDAV.
Cancelling a lesson works in Thunderbird and Evolution, but has no effect in DAVx5/Etar.
Substitutions do not work in any client, with different problems. In Thunderbird and Evolution, the original, recurring event disappears (in all weeks) and only the one-time substitution is visible. In DAVx5/Etar the recurring event remains (also the occurence that is substituted), leading to two parallel events (original and substitution).