[FIX] event_sale: confirm event

- Create an event with tickets
- Add the event in a SO
- Confim the SO
- Apply

An error occurs (record deleted).

Actually, the real error is hidden and is an insertion error in SQL on
`registration.editor.line` (`event_id` cannot be NULL).

`event_id` is mandatory but read-only, therefore it is not sent to the
server.

opw-787674

addons/event_sale/wizard/event_edit_registration.xml

@@ -10,7 +10,7 @@
                     please give details about the registrations</p>
                     <field name="event_registration_ids">
                         <tree string="Registration" editable="top" create="false" delete="false">
-                            <field name="event_id" readonly='1'/>
+                            <field name="event_id" readonly='1' force_save="1"/>
                             <field name="registration_id" readonly='1'/>
                             <field name="event_ticket_id" domain="[('event_id', '=', event_id)]" readonly='1'/>
                             <field name="name"/>