The ad service now returns ads matching the categories of the product that is
currently displayed. Changes in this commit:
- List all products' categories in products.json.
- Pass the current product's categories from the frontend to the ad service when
looking up ads.
- Store a statically initialized multimap from product category to ad in the ad
service.
- Return all ads matching the given categories when handling an ads request.
The ad service continues to return random ads when no categories are given or
no ads match the categories.
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