Ensure all relevant JS files are included after alphagov/govuk_elements#183 is merged

[robmckinnon]

A pending PR on the govuk_elements repo is going to add new javascript files, see here: alphagov/govuk_elements#183

We need to ensure these new javascript files are added to the gem.

Currently to add a javascript file to the gem, we create a new symlink to the file from govuk_elements, like in this example:

cd vendor/assets/javascripts/
ls -l
ln -s ../../../govuk_elements/public/javascripts/vendor/details.polyfill.js .
ls -l
cd ../../..
git add vendor/javascripts/details.polyfill.js

Using this approach we'd need to add each new javascript file separately. We could revisit this, and maybe symlink to javascript subdirectories from govuk_elements.

[robinwhittleton]

JS ended up being included in govuk_frontend_toolkit.