Remove "hent lenkje til kalenderfeed" from calendar feed button

Closes #228 (closed)

I've slimmed the remaining text down to just "Få hendingar i eigen kalender", because otherwise the text is so long the button often requires a separate line, and sometimes requires two lines inside the button. I think the new text essentially communicates the exact same message. Opinions on this? If we drop "app" here, I think we should drop "app" other places on the website, too.