Incorporate classnames package and do some general cleanup

[jakewheeler]

Summary

This PR does the following:

• Adds and makes use of classNames where appropriate
• Adds some reasonable defaults for prettier and eslint
  • A couple of new devDependencies were added for this
• Formats all applicable files using the new prettier defaults
• Ensures the Footer component renders a footer element rather than another header
• Moves NavigationLink and its prop type into its own file
• Moved the heavily used basePath variable into its own constants.ts file
• Fixed an error where Next.js couldn't find the favicon.ico
• Some other minor cleanup

Testing

1. Run npm i to install the new packages
2. Start the project (restart might be needed)
3. Make sure everything generally still works and looks the same

If you happen to run into any issues, it might be worth trying to delete the .next folder and restarting your dev build.

Discussion

The "reasonable defaults" felt like a good starting point to me but we can definitely adjust these over time if we think any of the ESLint rules aren't useful to us.