Product: add a location_count, user_count & proof_count

[raphodn]

Story

similar to #419

Queries

Location count

// product id
Price.objects.filter(product_id=product.id).values_list("location_id", flat=True).distinct().count()
// product category
Price.objects.filter(category_tag=category_tag).values_list("location_id", flat=True).distinct().count()

User count

// product id
Price.objects.filter(product_id=product.id).values_list("owner", flat=True).distinct().count()

Proof count

// product id
Price.objects.filter(product_id=product.id).values_list("proof_id", flat=True).distinct().count()