New Main Media Designs

So far this is just the immersive (`ArticleDisplay.Immersive`, all `ArticleDesign`s) main media, so we may not need to create sub-issues.

We'd like to reduce the size of the immersive main media image to bring the whole headline above the fold. The previous design said that part of the headline could be cut off by the bottom of the viewport.

Design can provide details on exactly how much spacing we need between the bottom of the headline box and the bottom of the viewport.