Add CSS fix for visually-hidden text

Borrowed from v5 of govuk-frontend which includes
the changes from this pull request to fix an issue
with how certain screen readers announce spaces in
visually hidden text:

alphagov/govuk-frontend#3836

app/assets/stylesheets/govuk-frontend/extensions.scss

@@ -59,6 +59,19 @@ $govuk-destructive-link-active-colour: $govuk-text-colour;
   @include govuk-link-style-destructive-no-visited-state;
 }
 
+// TODO: Remove when updating to v5 of govuk-frontend
+// Absolute positioning has the unintended consequence of removing any
+// whitespace surrounding visually hidden text from the accessibility tree.
+// Insert a space character before and after visually hidden text to separate
+// it from any visible text surrounding it.
+.govuk-visually-hidden::before {
+  content: "\00a0";
+}
+
+.govuk-visually-hidden::after {
+  content: "\00a0";
+}
+
 // GOVUK Frontend's grid-column classes are built from the $govuk-grid-widths map
 // that means we can extend the map to get extra classes
 $notify-grid-widths: (