Draft: refactor event feeds
My goal is to unify calendars and ICS feeds.
My first attempt was to use fullcaneldar's icalendar support. That requires a bunch of additional stuff though. I did not manage to get that working so I gave up.
My second attempt (this branch) is to change the event feeds so they can generate both ICS and JSON.
While working on that I also replaced django-ical by a solution that integrates better with generic views.