Price API: Allow filter on product categories tags

[TTalex]

Problem

I would like to fetch prices matching a specific product category, for example "en:hazelnut-spreads".

Currently, the API only allows filtering on category_tag for products without a barcode (and thus no product relation)

Proposed solution

Add a product_categories_tags__contains filter

[raphodn]

Ah, N+1 queries 😁
in the price views.py we already have a select_related so it shouldn't have too much of an impact.

But to be on the safe side I'd rather

• have this in a seperate PR
• add 1-2 tests that use assertNumQueries
• rename to product__categories_tags__contains to show the relationship

[TTalex]

I created said PR, but I could use your help writing the tests.