Add mega-dropdown component.

[zachj0hnston]

I'd love to see the mega-dropdown Khan Academy uses for the subjects tab added as a react component.

[ZeikJT]

Me too 👍

[zgotsch]

oops wrong button >_>

[zgotsch]

Anyhow, currently it's still implemented on the site using the jQuery plugin but if/when we transition it to React I'll make sure it ends up here.

[zachj0hnston]

Awesome sauce!