Define more ARIA landmarks in the CMS admin

[thibaudcolas]

Part of #4199 (comment). See also #4623 (WIP).

To explore the content of a web page, users of assistive technologies rely on the use of Landmarks to identify major parts of the web page, e.g: navigation, main, footer, form...etc.

Wagtail only has a couple set up right now, we should add more. ARIA roles we should use for landmarks: banner, navigation, main, contentinfo, search (https://dequeuniversity.com/assets/html/jquery-summit/html5/slides/landmarks.html)

A/C:

• CMS admin UI has ARIA roles defined for major parts of the UI: header, main menu, content, search, tabs, footer, etc

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Additional readings on the importance of proper heading structure and landmarks:

• Headings - Page Structure - WAI Web Accessibility Tutorials
• HTML 5 and ARIA Landmarks
• Improving access to landmark navigation - The Paciello Group

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We will want to do this for:

• header
• main menu
• main content
• search
• tabs
• footer

[thibaudcolas]

See #5290.

[thibaudcolas]

We didn't really cover tabs that much here – they were in the initial issue description, but these aren't actually landmarks, so I think they warrant a separate issue.