Implementing delete function for photo

- Implement delete function for photo; deleting photo remove database record and file on aws S3.
- Implement function to assign front_page_photo to kitchen
You are right; Photo#destroy will automatically call remove_picture! Also, it does not delete the reference if a particular photo is front_page_photo of the kitchen. As a solution I redefined the method like this(not sure if it's the best way):
```
  def destroy
    @photo = Photo.find(params[:id])
    @kitchen = @photo.kitchen
    @kitchen.front_page_photo.destroy if @photo == @kitchen.front_page_photo
    @photo.destroy
    redirect_to kitchen_photos_path(@kitchen)
  end
```