Add /design-system endpoint

This is where the representation of the design system will reside.
When the application runs in production (with DEBUG = False) the url
will return 404.

cloud/routes.py

@@ -102,6 +102,18 @@ def _homepage_context() -> dict:
         random_featured=random_featured)
 
 
+@blueprint.route('/design-system')
+def design_system():
+    """Display the design system page.
+
+    This endpoing is intended for development only, and returns a
+    rendered template only if the app is running in debug mode.
+    """
+    if not current_app.config['DEBUG']:
+        abort(404)
+    return render_template('design_system.html')
+
+
 @blueprint.route('/login')
 def login():
     from flask import request