Expand notifications to open incidents

[xcampax]

Problem statement
I would love to be able to receive a reminder about incidents that are in an unresolved state to ensure attention.

Expected behavior
For incidents that are neither "dismissed" nor "closed", send a periodic email reminder to the assigned users. Ideally, tie the time intervals to customizable SLA periods like for vulnerabilities.

Mock

Additional context
Add any other context about the problem here.