This updates the base template and page-level UI components (top header

bar, search, navigation, and sidebar) to better meet accessibility

standards.
We're now using more specific tags for many of thes elements:

The top bar now uses <header>
The navigation areas in the top bar use <nav>
The page sidebar uses a standard <div>, but is split into a

<header>, <div>, and <footer> for use with the mobile menu
Individual sidebars should now use <nav> or <section>, depending

  on what's appropriate for that sidebar
The main content area uses <main>

Some ARIA roles and labels have been added to parts of these components

in order to provide context to screen readers.
This is not comprehensive, but does improve what we've had quite a bit.

The page now has distinct landmark areas, which can be more easily

accessed when using assistive technologies.

Checked that the page continues to render correctly in all major

browsers.
Inspected the landmark areas using a browser extension, verifying the

landmark trees and labels.
Also inspected using Firefox's Accessibility tree.