feat(selling): wire Proforma Invoice into Sales Order form

- Create > Proforma Invoice dialog with naming series, print format and
  letter head selectors, and an item-wise pending-qty grid
- Proforma tab listing issued proformas with inline view/email actions,
  shown only once at least one proforma exists
- Register the client script and add the connections dashboard link

erpnext/hooks.py

@@ -38,6 +38,7 @@ web_include_icons = [
 
 doctype_js = {
 	"Address": "public/js/address.js",
+	"Sales Order": "public/js/sales_order_proforma.js",
 	"Communication": "public/js/communication.js",
 	"Event": "public/js/event.js",
 	"Newsletter": "public/js/newsletter.js",