[AWAITING REVIEW] Implementing #411 - public calendar sharing

The aim: implementing [public link-sharing of calendars](https://github.com/owncloud/apps/issues/411). The functionality that works:
 - link-sharing a calendar
   - displaying the link-shared calendar, along with events, info and download URL
   - downloading the shared calendar with the provided URL
   - password-protecting a link-shared calendar and password prompt;
   - accessing password-protected public calendars;
   - date-limit tested.
 - link-sharing a single event
   - sharing GUI in pure HTML/CSS, with as little JS as possible (using CSS transitions, etc)
   - password-protecting
   - date-limiting
   - displaying
   - downloading iCal.
 - new CSS/HTML interface for sharing calendars and events with users/groups on the same server
 - CSS-only tooltips
 - general fixes
   - information on event being shared publicly via a link-shared calendar has been added to event edit interface
   - changing the timezone now reloads calendar data (both in user's view, and in link-shared public view)
   - event datetime data are now adjusted to timezone
   - link-shared view of events and calendars allows the user to change the timezone data is displayed in.

The public link-sharing of both calendars and events can now be considered complete. Events can be individually link-shared regardless of their PRIVATE/CONFIDENTIAL status, but they will not show up (CONFIDENTIAL) or will only show as busy time (PRIVATE) in link-shared calendars.