Prepare app for working behind a proxy (nginx).

Adapt user form

common/utils/nginx_utils.py

@@ -0,0 +1,7 @@
+from flask import current_app, request, url_for
+
+
+def prefixed_url_for(endpoint, **values):
+    prefix = request.headers.get('X-Forwarded-Prefix', '')
+    current_app.logger.debug(f'prefix: {prefix}')
+    return prefix + url_for(endpoint, **values)