Add .js-hidden class to published-dates component

`gem-toggle` script in `govuk_publishing_components`  (https://github.com/alphagov/govuk_publishing_components/blob/master/app/assets/javascripts/govuk_publishing_components/lib/toggle.js) relies on a `js-hidden` class that was previously provided by `govuk_template`. We apply the properties at the component level to make sure the script still works as intended (revealing and hiding a list of previous publishing dates).

app/assets/stylesheets/components/_published-dates.scss

@@ -37,3 +37,8 @@
   padding-top: govuk-spacing(2);
   border-top: 1px solid $govuk-border-colour;
 }
+
+.js-enabled .app-c-published-dates .js-hidden {
+  display: none;
+  visibility: hidden;
+}