Surface Rendering Target On Interactive Articles

On all interactive articles DCR provides some minimal page structure: header, nav, footer etc. In addition, on interactive articles that aren't `ArticleDisplay.Immersive` DCR provides some additional page structure: headline, standfirst, meta etc.

This structure may be slightly different for apps articles rendered by DCR: no header/nav, different footer, different meta (with follow buttons). We may want to surface the rendering target (possibly even iOS/Android) on these articles so that interactive developers can make tweaks to handle these differences.

It may be useful to discuss further details with @HarryFischer