Initial port (and cleanup) of OST2.0 CSS

This PR aims to retain the look-and-feel of OST2.0 while keeping CSS and layout management under control.  This is done in the following ways:
- using partials to expose high-level structure in layouts
- creating a naming correspondence from partials to helpers to stylesheets
- using more explicit CSS classes and ids
- increasing the use of helpers to keep views simple
- minimizing CSS selector scopes to the intended portion of the page
Some legacy CSS (main.css.scss) and layout (junk.html.erb) information has been retained to reference, but is either not currently used or scoped to avoid collisions with newer code.

Currently there is support for Home, Dashboard, and Course Catalog top-nav links, mostly to exercise the current layout options.