Add GOV.UK Frontend JS

This includes the JS for all GOV.UK Frontend code.
If our frontend build includes a module bundler in
future, we should only include the JS for the
components we use, as with our Sass.

app/assets/javascripts/main.js

@@ -1,3 +1,5 @@
+window.GOVUKFrontend.initAll();
+
 $(() => $("time.timeago").timeago());
 
 $(() => GOVUK.stickAtTopWhenScrolling.init());