Add placeholder for nonce to Google analytics script tag

[JoeChapman]

So we can implement a Content Security Policy that does not allow unsafe-inline scripts to be loaded we should sign our script tags with a nonce.

A nonce can be generated in server middleware on each request using npm uuid, assigned to the locals object and the CSP script-src directive.

When the template is rendered the placeholder is substituted with the value of locals.nonce

If the value of the placeholder attribute (the nonce value we just set) and the value of the nonce on the server match then the request can proceed

Associated issues
UKHomeOfficeForms/hof#105
alphagov/govuk_template#258

[JoeChapman]

PR for script tag placeholder: alphagov/govuk_template#268