Return related artefacts grouped by their type of section

This will return related artefacts for an artefact in three groups:
those in the same subsection, those which exist in sibling subsections,
and any others from elsewhere on GOV.UK.

This is in preparation for changes to how internal related links
are displayed on GOV.UK.
Refactor related artefact grouping method

- When the artefact has no section, its related 
artefacts should be present in the "other" array
- Use the primary_section method to find the 
section instead of traversing the tag_ids array
- Use the parent_id from the primary tag instead 
of splitting the tag_id. If no parent tag exists,
any other related items will fall into 'other'.
Merge remote-tracking branch 'origin/related-artefact-grouping'