init: function(server_url, db, channel, options, rule) {
     this._super();
     options = options || {};
     _.defaults(options, {
         buttonText: _t("Chat with one of our collaborators"),
         inputPlaceholder: null,
         defaultMessage: _t("How may I help you?"),
         defaultUsername: _t("Visitor"),
     });
     user_session.setup(server_url, {use_cors: false});
     this.load_template(db, channel, options, rule);
 },