Markdown support

[felixrindt]

This is a very bold issue, but I don't know of any place (except ticket layouts) pretix core is using a rich text editor like pretix-pages does.

Also, from my experiences, I always had preformatted documents written in another editor I copied into pretix-pages which broke the formatting. Plain-text markdown would have been much nicer, although that might not fit non-technical users.