Website: re-implement layout using CSS Grid Layout

[ianmcburnie]

A fun exercise in learning CSS Grid Layout. We can redo the current DS4 (default) site, and start from scratch with the new DS6 site.

In terms of IE10 and 11, the website is a very simple layout, and therefore we should be able to implement the following fallbacks:

https://rachelandrew.co.uk/archives/2016/11/26/should-i-try-to-use-the-ie-implementation-of-css-grid-layout/

[ianmcburnie]

Also, the current MWEB experience for DS4 site is pretty sucky. We can address that too.

[ianmcburnie]

Probably not much point and going back and refactoring the ds4 site now, unless anybody wants to do so as a "fun" exercise. Sean updated the DS6 site to use CSS Grid Layout, so I'm going to close this issue.