Standardize the Jenkinsfile
Standardize the Jenkinsfile

- The Jenkinsfile here pre-dates `govuk-jenkinslib`.
- The only thing that the default library Jenkinsfile doesn't do is
  `vale` linting. We're using a [really
  old](https://github.com/alphagov/govuk-puppet/blob/master/modules/govuk_jenkins/manifests/packages/vale.pp#L11)
  version of Vale anyway (the current is 1.7.1), so I'm not sure of its
  value any more. If we want it, we can migrate it to be run on
  Concourse, or make it a pre-commit hook, or somesuch.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs