The property groups were causing us issues and making it very hard
to handle these properties. And since we were not dealing with them
anyway, it's better to just remove them (standard allows this).
ical.js doesn't deal with them correctly, which is the source of this
problem. More info:
https://github.com/mozilla-comm/ical.js/issues/411
At the moment it only shows recurrence in the calendar. You can't edit
specific instances, only the whole series (the first event). You can't
event edit the rrule at the moment. It's view only (mostly).
While this is not much, this is already a major boost to the usability
of etesync web.
Fixes#15.