Dev unavailable products

[vkrashenko]

add in productsAvailablityService two extra methods "isInStock" and "isBuyable". If product quantity = 0 but product can be purchased we can see "sold out". If product can`t be purchased we can see "unavailable".

[asvishnyakov]

• [x] Instead of https://github.com/VirtoCommerce/vc-storefront/blob/b7dfd45388f5c475f8db90f3babe6c2dc9f20546/VirtoCommerce.LiquidThemeEngine/Converters/ProductConverter.cs#L50-L51 write login the same as https://github.com/VirtoCommerce/vc-storefront/blob/b7dfd45388f5c475f8db90f3babe6c2dc9f20546/VirtoCommerce.LiquidThemeEngine/Converters/ProductConverter.cs#L40-L48 i.e.

  if (if (product.Variations != null && product.Variations.Any())
  {
      result.Buyable = product.Variations.Any(v => v.IsBuyable);
  }
  else
  {
      result.Buyable = product.IsBuyable;
  }

  How to test: make all variations and product unavailable. Then make one variation available. Product must be available on storefront.

• [x] Fix summary: https://github.com/VirtoCommerce/vc-storefront/blob/b7dfd45388f5c475f8db90f3babe6c2dc9f20546/VirtoCommerce.LiquidThemeEngine/Objects/Product.cs#L44-L51 https://github.com/VirtoCommerce/vc-storefront/blob/b7dfd45388f5c475f8db90f3babe6c2dc9f20546/VirtoCommerce.LiquidThemeEngine/Objects/Variant.cs#L40-L44 https://github.com/VirtoCommerce/vc-storefront/blob/b7dfd45388f5c475f8db90f3babe6c2dc9f20546/VirtoCommerce.Storefront.Model/Catalog/Product.cs#L89-L92
• [x] Add tests: https://github.com/VirtoCommerce/vc-storefront/blob/master/VirtoCommerce.Storefront.Test/ProductAvailabilityTests.cs