Filter markdown conversion

[jochenklar]

Rationale / Begründung

Created markdown output should be filtered using https://github.com/mozilla/bleach. Markdown conversion should be also deactivatable in the settings.

References / Verweise

• https://bleach.readthedocs.io/en/latest/clean.html
• https://python-markdown.github.io/change_log/release-3.0/#safe_mode-and-html_replacement_text-keywords-deprecated