[FIX] purchase: No drag and drop when grouped by Vendor

Fixes #47727
Closes #47955
Before this commit, it was possible to change the Vendor/Currency of the PO when Grouped by Vendor/Currency on Kanban view which is not Ideal Behavior.

Now we block drag and drop using these fields.

closes odoo/odoo#48050

X-original-commit: 7b19964a203c205b67d8f3bc05c1279362d38a61
Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>

addons/purchase/views/purchase_views.xml

@@ -434,11 +434,11 @@
             <field name="arch" type="xml">
                 <kanban class="o_kanban_mobile">
                     <field name="name"/>
-                    <field name="partner_id"/>
+                    <field name="partner_id" readonly="1"/>
                     <field name="amount_total"/>
                     <field name="state"/>
                     <field name="date_order"/>
-                    <field name="currency_id"/>
+                    <field name="currency_id" readonly="1"/>
                     <field name="activity_state"/>
                     <progressbar field="activity_state" colors='{"planned": "success", "today": "warning", "overdue": "danger"}'/>
                     <templates>