Add and use textarea component from GOV.UK Frontend

For the "Something else" branding form we want the form label to be the
title. This brings in the textarea component from GOV.UK Frontend in
order to do this since that contains code to set a the textarea label as
the page heading in an accessible way.

The rest of the textarea fields have not been switched to use the new
component yet.

gulpfile.js

@@ -72,7 +72,8 @@ const copy = {
         'checkboxes',
         'radios',
         'input',
-        'inset-text'
+        'inset-text',
+        'textarea'
       ];
       let done = 0;