Allow editing a calendar event metadata with a rich text editor

[lucaa]

Is your feature request related to a problem? Please describe or add steps to reproduce your use case.

Calendar event description can be edited today from the popup that is displayed in calendar view - create or edit event and standalone, on the calendar page.

In neither of these places the calendar description can be edited with a rich text editor (wysiwyg), even if the field itself is rich text and rendered as rich text.

Describe the solution you'd like

In order to fully benefit from the fact that a calendar event is a page and can be used with all the features of a page, the calendar event description should be editable with the wysiwyg editor, at least in the standalone view, so that users can add extra information, attachments, macros, etc.

In this case, the event view popup could display only the first x characters of the description, in order to avoid bloating the view, or a simplified version of the description.

Additional context
The need to have a complete description for an event is mentioned in #60 as well, although the solution proposed there is more complicated.