Adding vehicle-tax 'exception' page

We are changing the slug of tax-disc to vehicle-tax. This is one of the top five visited pages on GOV.UK so to mitigate against unwanted 404s I'm copying the tax-disc files (with minor CSS modifications - id names) and deploying them to production in advance of the slug change.

Once the new slug is up and running I'll remove the tax-disc files from here and update the various hard-coded links in other files.