Investigate how to render ad placeholder elements for interactives

Interactive articles can be quite different to standard articles so we can't insert placeholders every `x` block or every `y` paragraph since they do not follow a regular pattern or structure.

The team who create the interactive articles apparently sometimes put their own placeholders in the interactive code but this won't currently work with the way that apps placeholders are done in DCAR.

We need to consider how to feed the ad placeholder positions through the data model and enhancers rather than using the dynamic ad placeholder insertion function as currently done for apps articles.