[FIX] website_crm_partner_assign: get the lead to forward at save

When forwarding a list of leads to a partner, the lead_id was not send.
As the field `lead_id` is readonly, its value wasn't sent by the client
on save click.

This behaviour of not sending readonly fields in x2many has been correctly
implemented with the new views but some views (like this one) was basing
their behaviour on this breach.

See rev. odoo@3b3f6f0 for more details.

fix-feedback-saas16

addons/website_crm_partner_assign/wizard/crm_forward_to_partner_view.xml

@@ -17,7 +17,7 @@
                     </group>
                     <field name="assignation_lines" attrs="{'invisible': [('forward_type', 'in', ['single',False])]}">
                         <tree create="false" editable="bottom">
-                            <field name="lead_id" readonly="1"/>
+                            <field name="lead_id" readonly="1" force_save="1" />
                             <field name="lead_location" readonly="1"/>
                             <field name="partner_assigned_id"/>
                             <field name="partner_location" readonly="1"/>