Fix missing setting for tornado

Author: rgaiacs

binderhub/app.py

@@ -964,6 +964,7 @@ def initialize(self, *args, **kwargs):
                 "registry": registry,
                 "traitlets_config": self.config,
                 "traitlets_parent": self,
+                "template_file": self.template_file,
                 "about_message": self.about_message,
                 "banner_message": self.banner_message,
                 "extra_footer_scripts": self.extra_footer_scripts,