Remove node-sass and streamline menu

I noticed that node-sass was slowing down the CI because it gets compiled by node-gyp after npm install. I think it's worth removing it completely and converting the very little SCSS code we have to CSS modules. So that's what I've done, and I now import bootstrap.min.css in index.tsx (instead of bootstrap.scss in index.scss).

While I was converting the menu styles to CSS modules, I simplified a number of things in the components: I've removed the sub-components MenuButton and SignOutButton, as I don't think they're worth the extra abstraction level, I've added a small togglePanel function, and I've implemented an "active" state for the buttons.