Support single block mode on block lists

[sussexrick]

If a single block mode is enabled on a block list data type, the ModelsBuilder model will be an instance of BlockListItem<GovUkNotificationBanner, GovUkNotificationBannerSettings> (assuming the configured block type is a notification banner).

This is not an overridable model and not compatible with the partial view we render components with.

The ModelsBuilder model for a block list without single block mode is made overridable by OverridableBlockListPropertyValueConverter.cs so it is likely that the support would need to be added here.

However this could only ever return some generic interface because this property value converter has no knowledge of GovUkNotificationBanner or GovUkNotificationBannerSettings so it might also require another property value converter in GovUk.Frontend.Umbraco that inherits the one above and does more. There may also be impacts elsewhere, eg on the partial views that render components.