Replace type of `notification_type` field with type NotificationType in v4

[ryaplots]

Summary

Start using the NotificationType type everywhere in the backend, including for the notification_type field starting v4

Current Situation

We have a NotificationType type which we can't use in v3 for the notification_type because this would be a breaking API change. So we currently use strings (the enum converted to string).

Desired Situation

Use the NotificationType type across the backend.

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