Keep all calendar events

[Stealing the calendar from the website](https://github.com/artsmia/calendar-graph/blob/8cdadcf9e2a32295e3e618564144a8f5bd7d06d2/Makefile#L3-L8)
loses all past events. That's no fun. This takes all the old
`calendar.json` blobs and merges them.
Handle `full-calendar.json` from `make calendar`
No more `.instances` in json, strip the code too