[FIX] payment_paypal: missing Paypal link

When an invoice is sent by email, the Paypal link is not included,
although it is in the template.

This is because the `paypal_url` field has gone missing after Accounting
refactoring.

opw-686216

addons/payment_paypal/models/__init__.py

 # -*- coding: utf-8 -*-
 
+import account_invoice
 import paypal
 import res_company

addons/payment_paypal/models/account_invoice.py

+# -*- coding: utf-8 -*-
+
+from openerp import models, fields, api
+
+import werkzeug.urls
+
+class AccountInvoice(models.Model):
+    _inherit = 'account.invoice'
+
+    paypal_url = fields.Char('Paypal Url', compute='_compute_paypal_url')
+
+    @api.depends('type', 'number', 'company_id', 'currency_id')
+    def _compute_paypal_url(self):
+        for inv in self:
+            if inv.type == 'out_invoice' and inv.company_id.paypal_account:
+                params = {
+                    "cmd": "_xclick",
+                    "business": inv.company_id.paypal_account,
+                    "item_name": "%s Invoice %s" % (inv.company_id.name, inv.number or ''),
+                    "invoice": inv.number,
+                    "amount": inv.residual,
+                    "currency_code": inv.currency_id.name,
+                    "button_subtype": "services",
+                    "no_note": "1",
+                    "bn": "Odoo_Invoice_PayNow_" + inv.currency_id.name,
+                }
+                inv.paypal_url = "https://www.paypal.com/cgi-bin/webscr?" + werkzeug.url_encode(params)