Add to different calendars

[hell03610]

We currently give the option to add the event to a calendar, but it only works with google calendar. It can be interesting to support other calendars.

See:

• https://www.addevent.com/add-to-calendar-button
• https://litmus.com/blog/how-to-create-an-add-to-calendar-link-for-your-emails

As inspiration