Add ``icalendar`` requirement to the build.

https://github.com/madewithbytes/us_ignite/issues/106

This package allows to generate ics calendar events.
Add support to export calendar events.

https://github.com/madewithbytes/us_ignite/issues/106

The calendar events can be exported as an ``ics`` file.

This file is helpful to import into standalone calendar
applications.