Extend home helper to have two modes

One of these can be used for terse navigation links, and the other for
the masthead.
Rephrase dynamic pages

Includes helpful links on the error 404 page.
Implement basic partials and styling layout

This is raw CSS right now, but will probably go to SASS.
Switch out CSS for SASS

This clarifies the code quite a bit, adding an additional Grunt step.