DCAR: Toggle Display Of Ad Placeholders In Development

By default we don't show the ad placeholders that are used to insert ads; we only display them if we're able to check with the native layer that the user is not premium.

https://github.com/guardian/dotcom-rendering/blob/89015504a3457c41d5acda25c754d1e95f9a5366/dotcom-rendering/src/components/AdPortals.importable.tsx#L130-L155

As a result, when viewing DCAR in a browser for development purposes you will never see the ad placeholders, because there is no native layer to query.

However, sometimes it's useful to be able to see these placeholders outside of the app. For example, when interactive developers (Design, Visuals) are building interactives they would like to be able to see them, to make sure they can work around them correctly.

We discussed coming up with a mechanism for toggling the placeholders on and off for development purposes, possibly with a query parameter. Ideally this parameter would work in preview.