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());

gulpfile.js

@@ -114,7 +114,8 @@ const javascripts = () => {
       paths.npm + 'query-command-supported/dist/queryCommandSupported.min.js',
       paths.npm + 'diff-dom/diffDOM.js',
       paths.npm + 'timeago/jquery.timeago.js',
-      paths.npm + 'textarea-caret/index.js'
+      paths.npm + 'textarea-caret/index.js',
+      paths.govuk_frontend + 'all.js'
     ]))
     .pipe(plugins.uglify())
     .pipe(plugins.concat('all.js'))