remove more junk from EventCalendar.
implement and spec timespanlib/kalindar/event_calendar.rbspans?
specs, i.e. for Event.modifiable?
implement event.modifiable.
prepare extraction of timespan-based event/occurence consumption.
reflect changes, recuring events are shown, but cannot be edited.
do not show event edit link for not modifiable events.
add todo about modifiable events.