Redirect public view to public calendar ics endpoint if requested with Accept: 'text/calendar'

The server already does the request with the following header `Accept: 'text/calendar, application/calendar+json, application/calendar+xml'`. `text/calendar` isn't a MIME type requested by common browsers.

Closes https://github.com/nextcloud/server/issues/17746



Signed-off-by: Thomas Citharel <tcit@tcit.fr>