[Core API] Tag hierarchy

• GET /tags/:tag_id/children route to list all children of the provided tag.
• POST /tags/:tag_id/children route to create a child for the provided tag.
• PATCH /tags/:tag_id route can update the parent tag.
• Added fields updated_at, created_at to the Tag response body
• Added field children to the Tag _links

Deprecated

• Deprecate tag from PATCH /conversations/:conversation_id. Please use tag_ids instead.