Add "fixed" class

Added a "fixed" class so developers can style their fixed nav differently when it's position changes to fixed.

Since it gets removed from the dom and re-cloned each time it switches between fixed and static, there's no reason to remove the fixed class.
Clean up fixed class

I was wrong the first time. Apparently we do need to clean up the "fixed" class. That should do it.
Merge pull request #4956 from wjthomas9/patch-1

Add "fixed" class