Kalenderfeed -> kalenderapp-integrasjon

Renames "Kalenderfeed" to "Kalenderapp-integrasjon", and improves the button's text.

The text is almost excessively long now, but I think it's OK for a short transition period.

I considered making the button yellow, to bring extra attention to it for a period of time, but I think we shouldn't do that until we've finished upgrading the feed for now, i.e. #223 and #226 (closed).