Publish to the npm registry from this repo

We currently use https://github.com/alphagov/govuk_frontend_toolkit_npm to publish to https://www.npmjs.org/package/govuk_frontend_toolkit.

Ben (@easternbloc) has had a moan because this means that when you `npm install`, the toolkit is inside `./node_modules/govuk_frontend_toolkit/govuk_frontend_toolkit`, which is admittedly a bit silly.

This pull request is an attempt to appease him.

I have updated the govuk_frontend_toolkit_npm job on our CI server so that it runs this script on every push. When this pull request is merged, I expect:

- A new commit to be pushed to this repo because `VERSION.txt` and `package.json` are not in sync
- The updated package to be published to https://www.npmjs.org/package/govuk_frontend_toolkit

I think this will have the added benefit of the govuk_frontend_toolkit_npm job not failing every time the existing version fails to publish.