Move exercise description to markdown

[rolandgeider]

Since bleach is deprecated and won't be getting security patches much longer, we should remove the library and move the exercise's description to markdown.

TODO:

• find a python library to render and filter markdown. We still only wanto to allow a very simple formatting: bold, italic, paragraphs and lists ('b', 'i', 'strong', 'em', 'ul', 'ol', 'li', 'p')
• decide wether to convert the description to html in react/flutter or expose a rendered attribute over the api (this is cached anyway so it doesn't matter much if it's expensive). Also, think about backwards compatibility, what happens to the other consumers of the exercises
• convert current descriptions from html to markdown

https://bluesock.org/~willkg/blog/dev/bleach_6_0_0_deprecation.html

[rolandgeider]

Sub tasks

wger-project/flutter#470
wger-project/react#853