Rationalize the CSS for nav

The box that's front and center in the visual design has three levels
for nav. Rather than classing all of these levels as "buttons" and
distinguishing them as "tips," "nav," and the one-off "rip," now we
class them all as "nav," and then use "level-1," etc. for the three
levels.
Bump version to 7.1.3
Bump version to 7.1.3-dev