Fallback to $UWpref when it is not defined, which is most of the time.
When $UWpref contains non-alphanumeric characters, such as Han
characters on Chinese sites, $UW_file_path should contain the English
version of the URL.

Since responsive_tables_filter will only be in the responsive version of
the profile, it does not need the custom compatibility code which
allowed it to run on both responsive and non-responsive sites.