[IMP] mail: limit performance impact of the systray

To limit performance impact of the systray, we fetch by default only the last
1000 activities. That value can be configured with mail.activity.systray.limit
config parameter. Note that this can also improve a bit the readability of the
other activities view (for the standard activity view a pager is already
present).

Task-3562098

Part-of: odoo/odoo#139917

addons/mail/models/res_users.py

@@ -273,7 +273,9 @@ class Users(models.Model):
 
     @api.model
     def systray_get_activities(self):
-        activities = self.env["mail.activity"].search([("user_id", "=", self.env.uid)])
+        search_limit = int(self.env['ir.config_parameter'].sudo().get_param('mail.activity.systray.limit', 1000))
+        activities = self.env["mail.activity"].search(
+            [("user_id", "=", self.env.uid)], order='id desc', limit=search_limit)
         activities_by_record_by_model_name = defaultdict(lambda: defaultdict(lambda: self.env["mail.activity"]))
         for activity in activities:
             record = self.env[activity.res_model].browse(activity.res_id)