New Sidebar

Replaces the existing sidebar with a new, (kind-of) Angular-backed one.

The data for the sidebar is now seperate from the presentation, and located in `data/sidebar.json`. This hopefully makes it easier for other users to reorganize and add sidebar sections without having to dive into the HAML.

Additionally, instead of the previous `content_for :js_accordion` section on most docs pages, we apply the necessary classes by default on page load.

I attempted to replicate the existing styles to a degree, but my CSS/SASS skills are a bit rusty so I'd prefer to leave that to the pros.

Have not added back the animations, they seemed flashy and just made switching tabs slower.

In addition, this PR also adds Angular to the default set of JS loaded, so we can hopefully use it on other pages where needed.