Simple and optimized self_and_descendants query

What does this MR do?

The current Namespace#self_and_descendants queries traversal_ids and finds other namespaces with matching ancestors. This forces a subquery that confuses the PG11 optimizer.

To find self and descendants we only need to match the current node id inside the traversal_ids array. Without having to worry about ancestors we can avoid the subquery and avoid confusing the PG11 optimizer.

Old query:

SELECT "namespaces".* 
FROM "namespaces" 
WHERE (traversal_ids @> 
          (SELECT traversal_ids as latest_traversal_ids 
           FROM "namespaces" 
           WHERE (id = (9970))))

New query:

SELECT "namespaces".* 
FROM "namespaces" 
WHERE (traversal_ids @> ('{9970}'))

Does this MR meet the acceptance criteria?

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Related to #327224 (closed)