Investigate calendar modules and plugins for Redmine

Originally created by @sajolida on #7735 (Redmine)

As a replacement for #7521 (closed).

Redmine has a calendar module: http://www.redmine.org/projects/redmine/wiki/RedmineCalendar
But custom query can be saved on the calendar view, apparently.
There is also a iCal export plugin: http://www.redmine.org/projects/redmine/wiki/PluginICalendarExport
All this needs more research.

Edited May 15, 2020 by sajolida

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