Cleanup botathon/index.php navigation panel script

[Kenneth-W-Chen]

This change adjusts the code in the botathon/index.php page so that jQuery objects aren't continuously re-queried every time the page is scrolled. see 8951811

Other changes:

• Navigation panel classes are changed using native JS DOM selectors instead of jQuery because there's no reason to use jQuery in those cases. See 8951811, specifically, the changes to the function switchActive and the addition of the var navAnchors
• The initial state of the navigation panel is set properly. Previously, if the page's scroll position was set before the script could load (e.g., the user scrolls down and reloads the page, and the browser keeps the page scroll constant between loads), the active navigation anchor would always be the first one. See 04a3e43
• Refactoring. Renaming some variables and functions and changing scopes for variables and functions so that jQuery isn't continuously re-queried. Also changed to use let instead of var. See 7c81cfa