Misc a11y fixes

[alexgibson]

One-line summary

Fixes a handful of small things the a11y checker turned up.

Significant changes and points to review

• Adds role="contentinfo" to both old and new footer components.
• Fixes heading levels in old footer.
• Removes stray </div> from German /built-for-you/ page.
• Adds <main> landmark to new /about/ page.

Issue / Bugzilla link

#15004

Testing

• http://localhost:8000/en-US/about/ (new footer)
• http://localhost:8000/de/about/ (old footer)
• http://localhost:8000/de/firefox/built-for-you/

[reemhamz]