warning, warning, 2.13 advisory, calendar module changes

[sdetweil]

in 2.13 we changed over to the node-ical calendar parser to fix a bunch of problems, and add support for microsoft timezone names…

we didn’t know if this would impact other calendar modules, as we left in the ical (ical.js) calendar parser.

we intend to remove the ical parser on the next release…(Jan 1 approx)