Feature/take and store orders

`new` is going to receive an instagram photo ID in its `params` hash. For example:

1. User will click on <a href="/orders/new?instagram_id=1234">Buy this image</a>
2. Request will route to `orders#new` (this action)
3. `params` will look like `{instagram_id: 1234}` (note how the URL above yields this hash)
4. You can now use `params[:instagram_id]` to get the instagram_id passed to the URL (from the previous page).