404 page

First iteration of the 404 page, Fixes https://github.com/DoSomething/dosomething/issues/3289 

*Missing final title/copy for the default node*
See comment below for class naming. Would also recommend removing `div.` specifier here and just using class name.

(Actually, looking at the markup – is this necessary anymore? I don't see any `p` tags that would be affected by this.)