feat(NavExercise): finish basic desktop layout
feat(NavExercise): add secondary menu
feat(NavExercise): work on mobile responsiveness
feat(main.css): implement mobile nav
feat(NavExercise): complete slide in mobile nav
fix(NavExercise): fix positioning of copyright
feat(NavExercise): implement desktop nav mask
feat(NavExercise): add click event listeners for closing nav
feat(NavExercise): add chevron to mobile nav
feat(NavExercise): complete click handlers
fix(main.css): clean up CSS
fix(index.js): clean up JS
fix(NavExercise): clean up
fix(NavExercise): fix chevron alignment
feat(NavExercise): adjust secondary nav click handler
fix(index.js): collapsing secondary nav on click
fix(index.js): further impementation of click handlers
fix(index.js): fixed secondary nav event handler
fix(main.css): final touches
fix(index.js): revert previous click handler