Render slices in mobile view in Prismic Page Builder

[rcantin-w]

See this PR for context

We'd like to see if we can force the slices to render in a mobile-sized window. The current view makes it difficult to see what's actually going on, as well as grids affecting how much space the components take. The mobile view is more likely to be useful to editors.

[rcantin-w]

I jumped in a Prismic thread about this 😄 https://community.prismic.io/t/far-too-little-screen-real-estate-given-to-live-slice-previews-in-the-new-page-builder/15109/10?u=r.cantin