Query categories recursively

[Yelinz]

How should the category filter on the documents traverse the child categories to return all documents from parent and children.
Currently it will only do one layer of children

I think if we need to query categories recursively, we should either use something like MPTT, or cache the parents of each category in an ArrayField. Recursively iterating in a query is a bad idea performance-wise.
Since we didn't have positive-only experience with MPTT, I would suggest adding a all_parents = ArrayField(CharField()) to the category model, add a bit of code to keep them updated, and the query basically becomes all_parents__contains=my_category_slug

Originally posted by @winged in #365 (comment)

[czosel]

As of now, support for arbitrary nesting of categories is not something we plan to add to the frontend.