[bug] Notification verb is saved in database

[pandafy]

Describe the bug The notification verb is saved in the database. Thus, changing the verb in the NOTIFICATION_TYPE does not change the message of existing notifications.

Steps To Reproduce Steps to reproduce the behavior:

1. Create a default notification using the create_notification management command.
2. Go to Django admin and verify the notification message.

Default notification with default verb and level info by default

3. Change the verb for the default notification type https://github.com/openwisp/openwisp-notifications/blob/55a3443956f2ffa0301bd44a72d051f280fd02f7/openwisp_notifications/types.py#L7-L19
4. Go to Django admin again and verify the notification message

Expected behavior The notification message should show the updated notification verb

Possible solution

Update the notify handler to not save the verb in the database.

https://github.com/openwisp/openwisp-notifications/blob/55a3443956f2ffa0301bd44a72d051f280fd02f7/openwisp_notifications/handlers.py#L139-L148

Update the AbstractNotification model to use the verb from the configuration if database value is set to None

[shwetd19]

Hey @pandafy, I think the solution approach is to make the verb field nullable, by updating the handler to make verb optional, and add a get_verb() method that prioritizes NOTIFICATION_TYPES config while falling back to the database value, with checking.. notifications properly reflect updated verbs while maintaining backward compatibility can solve this bug

can I give it a try ?

[pranshustuff]

I kept the DB as fallback and made verb field nullable in the DB