Move Spine navigation section to template part

This will allow child themes to take over the generation of the
nav menu in the spine without having to also maintain the elements
of the body
Code formatting
Temporarily move navigation back to spine/body template
Introduce spine/site-navigation and spine/offsite-navigation templates
Remove arguments for built in menus that are defaults
Rename globally scoped variables for menu arguments

$site and $offsite lived in the global scope, which could cause
confusion in other places.
Update depth to 3 to meet WSU brand standards

See navigation here - http://brand.wsu.edu/media/web/web-standards/
Merge pull request #117 from washingtonstateuniversity/spine-nav

[PR] Move spine navigation to template parts

Introduces spine/site-navigation and spine/offsite-navigation templates which can be overridden by child themes