Redirect notifications to a slack channel for non-existent users

[rahmatrhd]

Summary
Currently, notifications are sent to slack IDs based on the email id in Grant.Owner or Appeal.CreatedBy. And since the ImportGrants feature, Grant.Owner is now possible to have empty value. In this empty owner case, notification won't be sent to anyone (only log.Error)

Proposed solution
Allow slack notifier to configure a fallback channel for non-existent users. This helps us to monitor orphan grants.

[bsushmith]

If this is based on orphan grants concept where Grant.Owner can be empty, can we think of alerting job/system to send one alert to a specified slack channel that there are orphan grants to look into.

Sending alerts at slack notifier level only for this feature for each grant separately - somehow doesn't seem right.

[rahmatrhd]

empty Grant.Owner is one case, another example is when user cannot be found or deactivated in Slack. This feature allows us as admins to have visibility on notifications that failed to be sent.

Given your idea, this also can be done by setting up an internal alerting when there is a failed notification. What do you think @AkarshSatija @mabdh 🤔