Fix incomplete regex escape function.

Use the regex from the TC39 [regex escaping
proposal](https://github.com/tc39/proposal-regex-escaping) instead of
our own, which didn't handle backslashes properly.

The code taken from the proposal is public domain (CC0 1.0 declaration),
so the attribution link is simply for maintainability rather than legal
reasons.

Fixes
https://github.com/alphagov/content-tagger/security/code-scanning/1.

app/assets/javascripts/filter-table.js

@@ -50,11 +50,9 @@
         GOVUKAdmin.redirect(link.attr('href'))
       }
 
-      // http://stackoverflow.com/questions/3446170/escape-string-for-use-in-javascript-regex
-      // https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/regexp
-      // Escape ~!@#$%^&*(){}[]`/=?+\|-_;:'",<.>
       function escapeStringForRegexp (str) {
-        return str.replace(/[-[\]/{}()*+?.^$|]/g, '\\$&')
+        // https://github.com/tc39/proposal-regex-escaping
+        return str.replace(/[\\^$*+?.()|[\]{}]/g, '\\$&')
       }
     }
   }