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

@cptvitamin Here's the header template for the new edX Platform Header
The associated changes for the Drupal Header and Footer are on qa.edx.org
The changes for the edX platform header are on steve.m.sandbox.edx.org
If you notice anything odd in the header or footer feel free to comment in line or just on the PR itself.