Set the URL prefix on the logout page

server/http.go

@@ -250,6 +250,7 @@ func (h *Server) handleLogout(w http.ResponseWriter, req *http.Request, session
 
 	data := map[string]interface{}{
 		"CSRFField": csrf.TemplateField(req),
+		"URLPrefix": h.urlPrefix,
 		"Services":  svcs,
 		"IsPOST":    false,
 	}