[WIP] REFACTOR: Re-work HTML structure and CSS

[choldgraf]

This is a refactor of our HTML and CSS, with the goal of improving a number of the UI/UX issues on the book theme, tightening up our white space and sizing, using a more sensible set of CSS classes that will make the theme behave more predictably, and adding more functionality for the in-page TOC on mobile widths.

It is WIP and will need to be rebased once #469 is merged in. Posting here now to track ongoing progress.

Major improvements

• We now use sticky-top to place our sidebar and topbar, rather than fixed, which means it will flow more naturally if things like banners are placed at the top of the page.
• Sidebar/topbar/toc font is slightly smaller than the main text font to create more space
• There's more space for the sidebar + toc bar
• Increased padding for the main content, and improved padding behavior on wide screens + with sidebar collapsed
• Programmatic definition of our margin size and placement, so that there's less hacky manually-chosen numbers in there
• The right sidebar is now viewable on mobile, via a button to toggle the toc similarly to the left sidebar
• The topbar buttons are now divided into two div groups (left and right) and use flexbox to position and center them nicely

closes #392
closes #459 (I think?)
checks off the last two boxes in #416

[choldgraf]

closing in favor of #472