[feature] Allow "filter can no longer be found" notifications to send to a different URL

**Version and OS**
Hosted

**Is your feature request related to a problem? Please describe.**
I've got notifications set up where others can see them, and they could care less when a bot fails and needs help. However, I definitely care about this! There's no way I can see to allow the "filter can no longer be found" alerts to go to a different endpoint than the normal notifications.

**Describe the solution you'd like**
When the "Send a notification when the filter can no longer be found on the page" checkbox is checked, show a "Notification URL List" specific to these notifications.

Start by tracing the existing notification URL configuration and the path that sends “filter can no longer be found” alerts. Add a separate Notification URL List controlled by that checkbox, and verify that these alerts use the separate endpoints while normal notifications remain unchanged.

Written by the indexing model from the issue text.