Markdown Form Field and Widget

[felixrindt]

Is your feature request related to a problem? Please describe.

We have multiple places (event description, notifications, ...), where we render markdown, but that is not easily discoverable by users.

Describe the solution you'd like

I propose implementing a custom field (model and form) and widget that allows preview (like Github or pretix) and has rendering functionality, maybe also considering other feature requests like #1512.

Additional context

We might also think about support for highlighting placeholder substitutions, e.g. in the context of writing notification text templates.