Fix missing skiplink target on product page

We didn’t have an element with an `id` of `content` on the product page
because we didn’t want the styles that come with it (from GOV.UK
Elements here: d6226bd2c8/public/sass/elements/_layout.scss (L9-L20))

This meant that the skiplink didn’t work on the product page because
it’s target was not on the page. The skiplink’s target is hardcoded to
point at `#content`.

The proper way to fix this would be to not hardcode the skiplink to
point at `#content`, ie what this pull request does:
https://github.com/alphagov/govuk_template/pull/275

Until that is merged, we can hack around the problem by overriding the
styles that Elements applies to `#content`, which is what this pull
request does.

app/assets/stylesheets/app.scss

@@ -249,3 +249,7 @@ details .arrow {
 .block-label input[disabled] {
   opacity: 0.5;
 }
+
+#content.override-elements-content {
+  padding-bottom: 0;
+}