merchant can upload images one at time

@cgruppioni @aslawter 

Right now only one image can be added at a time.
This is how vim has been automatically indenting. Our other controllers ar formatted the same way. This is taken from out products_controller:

```ruby
  def product_params
    params.require(:product).
      permit(
        :name,
        :description,
        :condition,
        :category_id,
        :price,
        :auction_starts_at,
        :auction_ends_at,
        :url,
    )
  end
```