Better support for Studio Grid Templates

Studio now allows us to organise our sections in grids that are no longer a strictly vertical stack of sections, but can have more interesting grids.

This task is to re-write the preview mode HTML boilerplate so that it has many of the same features like section names, without putting a parent element around a section which affects how it behaves as a grid child element.

Tip: perhaps we can use anchor positioning or pseudo elements.