Mockup rwd

[tanyeah]

Fixes #21

• Number of columns increase / decrease based on device width
• No drastic changes made to sidebar in the name of consistency (since the styles for that come from stylesheets used site-wide on w3.org)

[deniak]

@tanyeah MR looks good but I was wondering if we shouldn't center the ul#container to avoid having large right margin in some situations (see below). FWIW, adding justify-content: center would do the trick.

[tripu]

@tanyeah, apart from @deniak's idea, which I like, it looks good to me. Thanks!

I'm changing some of the CSS classes as I go making changes on my own branch, but I'll be careful to change them everywhere if that is so.