Move back-link component from GOVUK Frontend

`node_modules` isn't included on live so all
macros need copying across into the repo' code.

This adds the back-link component to the list of
macros to be copied across in `gulpfile.js`.

It also adds the files copied across as a result
of running the frontend build into the repo'.

gulpfile.js

@@ -62,7 +62,8 @@ const copy = {
         'template',
         'skip-link',
         'header',
-        'footer'
+        'footer',
+        'back-link'
       ];
       let done = 0;