[WIP] Implementing #411 - public calendar sharing

This is by no means final nor considered stable. The only functionality that works is:
 - link-sharing a calendar
 - downloading the shared calendar with URL -- use the URL provided by ownCloud upon sharing, add &download at the end.

Displaying the shared calendar is not implemented yet, hence accessing the vanilla URL directly will get you a 501 Not Implemented. Similarly, accessing password-protected public calendars is not implemented yet.