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
Note that

```ruby
kitchen.front_page_photo = some_photo
```

_won't_ save the `kitchen` record to the database.  You'd have to still call `kitchen.save`.

Still, I'm imagining the controller code to handle this.  You should be issuing a `PUT /kitchens/:kitchen_id` request with, e.g., `params == { front_page_photo_id: "123" }`.  Assuming you have the correct validations on the `Kitchen` model, i.e., you can't assign a front-page photo to a `Kitchen` unless that photo is one of the `Kitchens` photos, then your `KitchensController#update` action shouldn't have to change at _all_ to account for `front_page_photo_id`.  As far as its concerned, it's just another "update" request that happens to contain the `front_page_photo_id` param.