Reference ancestor_hierarchies in depth instead of ancestors

[weilandia]

This is helpful because we can more easily eager load ancestor_hierarchies than we can ancestors to avoid n + 1 queries.

tag = Tag.includes(:ancestor_hierarchies).first
tag.depth

[seuros]

Sounds good, can you add a changelog entry ?

[weilandia]

@seuros long delay here :) but I added to changelog. Can we could get this pr merged?