[IMP] crm_partner_assign: add field assigned_partner_id in view

website_customer filters the partners based on assigned_partner_id (the list of
customers that were impletemented by another partner). This field was not
visible so no way to actually display a partner in this list.
Fixes #430, opw 619254

addons/crm_partner_assign/res_partner_view.xml

@@ -111,6 +111,7 @@
                             <separator string="Partner Activation" colspan="2"/>
                             <field name="grade_id" widget="selection"/>
                             <field name="activation" widget="selection"/>
+                            <field name="assigned_partner_id"/>
                             <field name="partner_weight"/>
                         </group>
                         <group>