Add support for fullcalendar json feed (s)

[tothewine]

fullcalendar has support for getting events from a feed url. I suggest this method be used directly to populate the calendar bypassing the need for intermediate json conversion in client code. this can enable fine-grained control over presentation of the calendar by allowing manipulation of events (using customized class) without having to customize client code.

reference: https://fullcalendar.io/docs/v3/events-json-feed