Add split previews for web, tablet and mobile

[edwinmp]

The DI website chart pages have a provision for web, tablet and mobile previews ... it'd be great to have this included in our template repo so we can test our charts on these different device sizes as we build.
This should be implemented conventionally, with buttons that switch between the views.

[ThatcherK]

There is already a PR here where I started work on this, you can either build on it, or pick some bits and pieces, and start from scratch... Also take into consideration some of the feedback on the PR