Group related artefacts by section and subsection

Use the new related_artefacts_grouped_by_distance
method from Artefact to populate related
artefacts in the response.
Fix issue with travel advice additional related items
Respond with related items as an array, with group

Instead of returning a hash of groups for related items, continue to
return a single array of all related items, each with a 'group' value
corresponding to the related item group it is in (subsection, section or
other).

This maintains backwards compatibility with clients which already
consume the content API.
Bump content_models to 5.10.0

This adds the related_artefacts_grouped_by_distance method used in
this branch to retrieve artefacts in groups.
Merge remote branch 'origin/master' into grouped-related-links
Use grouped artefacts for travel advice
Merge remote-tracking branch 'origin/grouped-related-links'