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'.