Add a hiding topbar w/ scroll

[choldgraf]

This factors out part of #169 into just the navbar at the top so that we can iterate step-by-step.

This PR does a general refactor of the page layout in order to try and make it a bit more modular, so that at a top-level it's clear where each thing is spaced it. It also uses this as an opportunity to refactor the top-level bar so that it takes up less whitespace, has its own div for the buttons, and shows itself if you scroll up from down on the page. Finally, this also re-implements the "sidebar popout" functionality we saw before where you can write notes in the margins.

Topbar notes

All of the top bar buttons (toggle sidebar + the interact buttons) live in their own div. This will hide itself on scroll down, and show itself on scroll up. This means you can re-show these buttons from anywhere on the page if you like. Just scroll up.

General layout notes

This shouldn't change the general look and feel of the page, but things should just feel a little bit tighter now. The biggest difference currently is that the screen width is much wider, not sure if we want to keep that or not. Finally, tags will now automatically be added as classes to code and markdown cells. Any tag will be added as a class with the structure tag_{{ tagname }}.