Hide global header bar when a survey exists
Hide global header bar when a survey exists

It was either this hack or mess with the default govuk page template.
`app/server/templates/page-components/govuk_template.html` is copied
directly from `node_modules/govuk_template_mustache/...`.

I had originally been editing the govuk_template, but my changes
were getting overwritten by the build step.

Keeping the template as-is means we can avoid the problem of having
to manage overwriting + manually copying changes into the default
govuk template.

alphagov/spotlight,alphagov/spotlight,alphagov/spotlight