Support calendar sharing via MacOS Calendar.app

Currently, trying to share a calendar using the MacOS Calendar.app produce the following error:

"user":"ncadmin","app":"webdav","method":"POST","url":"/remote.php/dav/calendars/ncadmin/3F283E41-2239-452D-BE13-382E6CBB282F/","message":"Exception: {"Message":"HTTP\/1.1 501 There was no plugin in the system that was willing to handle this POST method.","Exception":"Sabre\\DAV\\Exception\\NotImplemented","Code":0,"Trace":"#0 \/var\/www\/nc12\/3rdparty\/sabre\/dav\/lib\/DAV\/Server.php(254): Sabre\\DAV\\Server->invokeMethod(Object(Sabre\\HTTP\\Request), Object(Sabre\\HTTP\\Response))\n#1 \/var\/www\/nc12\/apps\/dav\/lib\/Server.php(253): Sabre\\DAV\\Server->exec()\n#2 \/var\/www\/nc12\/apps\/dav\/appinfo\/v2\/remote.php(33): OCA\\DAV\\Server->exec()\n#3 \/var\/www\/nc12\/remote.php(162): require_once('\/var\/www\/nc12\/a...')\n#4 {main}","File":"\/var\/www\/nc12\/3rdparty\/sabre\/dav\/lib\/DAV\/Server.php","Line":487,"User":"ncadmin"}","userAgent":"Mac+OS+X/10.12.5 (16F73) CalendarAgent/386","version":"12.0.0.29"}

SabreDAV already support this->http://sabre.io/blog/2016/sharing-changes/ and would be nice to see this implemented into NC.

Start at apps/dav/lib/Server.php and appinfo/v2/remote.php, then trace the failing POST through 3rdparty/sabre/dav/lib/DAV/Server.php. Read the linked SabreDAV sharing changes and identify the integration points needed for MacOS Calendar.app sharing; done means the request no longer returns HTTP 501 and calendar sharing works.