add a switch to disable markdown for the whole app

src/strass/strass_app/forms.py

src/strass/strass_app/locale/en/LC_MESSAGES/django.po

src/strass/strass_app/locale/fr/LC_MESSAGES/django.po

src/strass/strass_app/migrations/0003_auto_20201028_1803.py

src/strass/strass_app/models.py

src/strass/strass_app/templates/strass_app/setup.html

src/strass/strass_app/templatetags/strass_tags.py

src/strass/strass_app/utils.py