This migrates the content structure of all pages to flexbox. As a result,
the subnav issues in the docs pages are fixed (no floating subnav over the footer).
It turns out that migrating the header and footer to flexbox is not strictly necessary
to complete before this patch. However, we can still simplify the DOM and fix up a lot
of the styling by doing so.
Considering that this patch impacts all pages on the site, I request that all reviewers
actually run the test plan as it's not feasible for me to test on multiple versions of
multiple browsers.
Depends on D7399