[2.2] Edited notifications reappear to users who have dismissed them and cannot be dismissed

Steps to reproduce:

Use the branch at: lp:~mpontillo/maas/iprange-notifications--bug-1393936
    (if it has not already been merged[1])

 - Create a large IP range sufficient to cover the subnet and cause an IP exhaustion warning
 - Dismiss the notification
 - Save any IP address or edit or delete any IP address associated with the same subnet

Expected behavior: no notification reappears, since it has been dismissed for that subnet.

Actual behavior: notification appears, but cannot be dismissed.

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Triage found that when the notification text changes, the trigger notices and send an event to the websocket handler. This causes a notification to be sent to the UI, and the dismissed event is "resurrected". However, dismissing it again is a no-op, since it has already been dismissed.

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[1]:
https://code.launchpad.net/~mpontillo/maas/iprange-notifications--bug-1393936