Bump Slimmer to 3.19.0
Pass the primary section for travel advice through to Slimmer

The content api now returns the section tags from
the travel advice index on each travel advice edition.

This changes the previous behaviour to pass this through
to Slimmer whilst still setting the stub section
for travel advice for breadcrumbs.
Only remove the first section tag

If there isn't a first section tag present, still append the stub section into the
tags array.
Merge remote-tracking branch 'origin/set-travel-advice-artefact-tags'