If the survey configuration does not itself have a template
we will use the one in etc/defaults/templates/sequence.html.njk.
It is not very likely that the template will be changed all that
often and it avoids issues when people forget to copy it across
to a new survey, etc.
If the `download` or `d` query parameter is supplied (even
without any value), the response will include a
`Content-Disposition: attachment` header. A filename will
also be suggested.
