Updating the platform header styles to match new site styles.

These changes are for the Header Styling story: https://openedx.atlassian.net/browse/ECOM-65
Which is related to the Drupal Front End story: https://openedx.atlassian.net/browse/ECOM-20
* Minor changes to navigation.html to place the Register button on the right side of the page.
* Updated SASS for the header to use new styles.
* Does not impact the course-level header (as intended) since it uses a 'slim' class to separate its styling.
* Introducing new variables specific to the navigation section of the header.
* Renamed links for Courses and Schools to align with Drupal site. (Will require new translations)

@wedaly @dianakhuang Dev CR
@talbs @andywaldrop Design review
@griffresch Product Review
Available at steve.m.sandbox.edx.org