fullcalendar 7.0 upgrade

[captn3m0]

7.0 has had beta releases: https://github.com/fullcalendar/fullcalendar-workspace/releases, which we are upgraded to already.

once the 7.0 stable drops, ICAL.js 2.x will be supported, and we might be able to drop our in-tree fork of @fullcalendar/icalendar, depending no whether the per-event color feature is fixed.