[IMP] event: allow to archive registrations

Event users cannot unlink registrations. To help them in their daily job
we added an active field on registrations allowing to archive them
without unlinking them.

Task ID-2204364
COM odoo/odoo#57022
ENT odoo/enterprise#13984
UPG odoo/upgrade#1897

addons/event/models/event_registration.py

@@ -21,6 +21,7 @@ class EventRegistration(models.Model):
     event_ticket_id = fields.Many2one(
         'event.event.ticket', string='Event Ticket', readonly=True, ondelete='restrict',
         states={'draft': [('readonly', False)]})
+    active = fields.Boolean(default=True)
     # utm informations
     utm_campaign_id = fields.Many2one('utm.campaign', 'Campaign',  index=True, ondelete='set null')
     utm_source_id = fields.Many2one('utm.source', 'Source', index=True, ondelete='set null')