Support rich text event descriptions

[tcitworld]

In the RFC Event Publishing Extensions to iCalendar there's a Styled-Description property. We could have a very basic editor based on tiptap.

• check that it doesn't cause issues with ICAL.js
• check support in calendar-js
• add support here

https://datatracker.ietf.org/doc/html/rfc9073#section-6.5

[st3iny]

We could also do Markdown instead of HTML.

Editing and rendering Markdown is well supported in the Nextcloud ecosystem already via @nextcloud/vue's NcRichContenteditable and NcRichText. It is also possible to embed the shared text editor from the Text app which also outputs Markdown.

Furhtermore, Markdown has the advantage of being legible as plain text as well. So we don't really need to declare an alternate representation in iCal (ALTREP).