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
This belongs in an ActiveRecord `after_create` callback.  Something like...

```ruby
class Photo < ActiveRecord::Base
  after_create :set_as_front_page_photo_if_first

  private

  def set_as_front_page_photo_if_first
    kitchen.update!(front_page_photo: self) if kitchen.photos.count == 1
  end
end
```

`update!` with the bang `!` will raise an error if something fails whereas `update` without the bang will only return `false`.  This is almost certainly what you want, since something has gone terribly wrong if `kitchen.update!` fails.