[FIX] event_sale: Expected attendees for event

When making a registration for event tickets from a SO, by clicking on apply
from registration.editor.form view, duplicated event registrations
were created(in function action_make_registration) because the registration_id
was not sent to the server.

opw:1824952

addons/event_sale/wizard/event_edit_registration.xml

@@ -11,7 +11,7 @@
                     <field name="event_registration_ids">
                         <tree string="Registration" editable="top" create="false" delete="false">
                             <field name="event_id" readonly='1' force_save="1"/>
-                            <field name="registration_id" readonly='1'/>
+                            <field name="registration_id" readonly='1' force_save="1"/>
                             <field name="event_ticket_id" domain="[('event_id', '=', event_id)]" readonly='1'/>
                             <field name="name"/>
                             <field name="email"/>