If `Confirmable#confirmation_sent_at` is equal to `0.days.ago`, then
`confirmation_period_valid?` will be deemed valid even if the setting is
configured to disable this outright. To prevent this error, we explicitly
check the configuration setting to be `0.days.ago`.
Call send_on_create_confirmation_instructions in after_commit instead of after_create, I think this is no harm in general and it makes things like async job work.
Fix#4062