This fixes the extremely poor accessibility of the old system, making it
possible to navigate the sidebar by keyboard, and also implicitly gives the
sidebar items the correct ARIA roles.
The source sidebar has a setting to remember whether it should be open or
closed. Previously, this setting was handled in source-script.js, which
is loaded with `defer`, meaning it is often run after the document is rendered.
Since CSS renders the source sidebar as closed by default, changing this
after the initial render results in a relayout.
Instead, handle the setting in storage.js, which is the first script to load
and is the only script that blocks render. This avoids a relayout and means
navigating between files with the sidebar open is faster.
