When plus icon is pressed, you would do: 
- Make an Ajax petition to ```CustomersController#new```
- Respond with html **modal** from this controller
- Finally render this respond on ``` #customer-modal``` id

Tips: You would reuse the **views/customers/_customer_data.html.haml** partial instead of redefine the sames fields in **app/views/invoices/_new_customer.html.haml**