Allow markdown in session and speaker descriptions

[jandd]

Is your feature request related to a problem? Please describe.
It is not possible to enrich speaker and session descriptions with links or other layout elements currently.

Describe the solution you'd like
A (potentially stripped down) markdown support would help with the issue.

Describe alternatives you've considered
Adding HTML support would be another option, but markdown would be easier to write. There are available markdown parsers for Python, so we do not have to reinvent one.

https://pypi.org/project/django-markdownfield/ seems like a good candidate.