Add layouts for left-side interactive [#49877749]

This turned out to be a lot easier than I'd expected, thanks to Upstatement's modularization of the CSS. This commit just has the runtime style changes, not the authoring.

https://www.pivotaltracker.com/story/show/49877749
Edit-mode for interactive-right layouts [#49877749]

This doesn't get the column widths right in edit mode, but it does put the columns in the right order (interactives left, embeddables right) when such layouts are selected.

https://www.pivotaltracker.com/story/show/49877749