Reorganize SASS files and begin implementing flexnav jQuery plugin.

- Split SASS files up into mobile and desktop files.
- Use flexnav to make mobile navigation more compact.
Stop using flexnav due to multi-level menu support, use custom jQuery.
Mobile menu toggles full expanded menu with no accordion; too many possible behaviors when every level is linked.
Make items navigation more compact on mobile devices and other adjustments.

- Add menu icon to mobile navigation.
- Add arrow icon to show menu levels.
- Make responsive grid units more consistent.
- Add option to adjust site title colors.