Persistent .ical urls - an .ical feed

[phansch]

Thanks to @mamhoff for suggesting this last week!
Instead of providing a static file for each download, we should provide a feed for the /calendars/:token/feeds/ical url. The first version of this would have one .ics file per calendar that is updated every time there is an update to the calendar.

(There would also be an /calendars/:token/feeds/rss url for RSS feeds, once #54 is done)

As far as I know, we don't have to push updates to clients. All clients check for new versions of a feed on their own. Correct me if I'm wrong here.