Share calendar to a circle.

Changes in the interface to allow:

* Calendar to be shared with a circle
* Show calendars (and events) that are shared with current user's circles

This need that the commit f45624f1819833086368d95f0173bd55a3fb8dae is applied in nextcloud core.

Signed-off-by: Vinicius Cubas Brand <viniciuscb@gmail.com>