Add URLs without slash at start of string

Due to an error in Specialist Publisher, we were sending URLs without a
slash at the start as the ID to Rummager. The sitemap uses these IDs to
build all the URLs and if they start with a slash assumes they are local
and prepends https://www.gov.uk to them.

This commit changes the check to do this unless the URL starts with
http, in which case it is an external URL. Previously this assumed that
if it started with a slash, it was an internal URL and appended the
slash.

Ticket: https://govuk.zendesk.com/agent/tickets/856504