[FIX] web: don't duplicate alert while backup

When you backup 3 dbs with the database manager, you had 3 times the warning.

Underscore is not loaded, so _.once not available.
This commit closes #13173

addons/web/views/database_manager.html

@@ -32,7 +32,9 @@
           var modal  = $(this).parentsUntil('body', '.modal');
           if (modal.hasClass('o_database_backup')) {
             $(modal).modal('hide');
-            $('.list-group').before("<div class='alert alert-info'>The backup may take some time before being ready</div>");
+            if (!$('.alert-backup-long').length) {
+              $('.list-group').before("<div class='alert alert-info alert-backup-long'>The backup may take some time before being ready</div>");              
+            }
           }
       });
   });