Add Markdown support for Articles

[disperate]

Description of problem
Currently it's not possible to format articles. This is unfortunate as longer text are hard to read and there is no possibility to add images or other kind of media.

Solution
Add support for markdown.

Additional context
TBD: How is preview mode integrated?

Criteria for acceptance

• validate markdown to prevent xss
• preview mode
• formatting bar
• display markdown instead of simple text