Update tests for change in lesson markup

Update tests to test for a link instead of a button when starting
challenges.
Flatten button styles globally
Tweak navbar

Adjust spacing for navbar buttons and add dividers.
First draft of color and font scheme

Update Foundation variables to match initial styling concept. Add
styles to top nav.
Fix 'Get Started' button position
Change alert color
Tweak fonts
Polish dashboard

Change dashboard to a 3-column layout. Apply initial styling.