Werk #16612: notifications: Fix plugin permissions not loaded automatically

Previously, it was possible for users with the "Notification configuration" permission to edit notification rules even if they did not have the permission to the plugin that was being used in the rule. When such a user edited such a rule, they were able to overwrite the notification plugin with any plugin they were allowed to use.

This werk stops users from editing rules that use notification plugins they don't have access to.

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