Add JavaScript to enable the use of PrimaryLists

One of the features implemented in [govspeak][1] is to allow long lists
to be collapsed to only show the first `n` items. This JavaScript makes
the frontend behaviour possible.

This JavaScript is already in [alphagov/specialist-frontend][2] but is
needed in [alphagov/specialist-publisher][3]. Putting it here saves duplication.

[1]: https://github.com/alphagov/govspeak
[2]: https://github.com/alphagov/specialist-frontend
[3]: https://github.com/alphagov/specialist-publisher