fix: run open edx behind a web proxy

Without the ENABLE_PROXY_FIX setting, running Open edX behind a web
proxy leads superset to redirect to http, and to the lms login page.

tutorcairn/templates/cairn/apps/superset/superset_config.py

@@ -7,6 +7,12 @@
 SECRET_KEY = "{{ CAIRN_SUPERSET_SECRET_KEY }}"
 SQLALCHEMY_DATABASE_URI = "postgresql+psycopg2://{{ CAIRN_POSTGRESQL_USERNAME }}:{{ CAIRN_POSTGRESQL_PASSWORD }}@cairn-postgresql/{{ CAIRN_POSTGRESQL_DATABASE }}"
 
+{% if not ENABLE_WEB_PROXY %}
+# Caddy is running behind a proxy: Superset needs to handle x-forwarded-* headers
+# https://flask.palletsprojects.com/en/latest/deploying/proxy_fix/
+ENABLE_PROXY_FIX = True
+{% endif %}
+
 # Languages
 # https://github.com/apache/superset/blob/dc575080d7e43d40b1734bb8f44fdc291cb95b11/superset/config.py#L324
 available_languages = {