Delivery status field should show by_bounces choice in choices.

Currently, the UserPreferences form has only two choices for delivery_status field, by_user and enabled. When a delivery is disabled due to bounces, the field's value is set to by_bounces which isn't visible in the UI making it impossible for the user to determine why their delivery isn't working as the field will appear "unset", which is indistinguishable from the default state of the field.