docs: def validate_template_name(self, key, value):
          """Validate template name.
  
          :param key: The template path.
          :param value: The template name.
          :raises ValueError: If template name is wrong.
          """
          if value not in dict(current_app.config['PAGES_TEMPLATES']):
              raise ValueError(
                  'Template "{0}" does not exist.'.format(value))
          return value