Rich markdown editor

[krawieck]

https://user-images.githubusercontent.com/10037914/186002239-f76d6b43-788b-4037-af6d-c4603d680f33.mov

closes #281

• [x] bold
• [x] italics
• [x] link
• [x] image
• [x] user tagging
• [x] instance tagging
• [x] header (6 levels!)
• [x] strikethrough
• [x] quote
• [x] list
• [x] code
• [x] superscript
• [x] subscript
• [x] spoiler