[TASK] Migrate backend scaffolding to flexbox

Use flexbox for layouting to avoid transform/position
based shifting of the content module.
Also use `position: sticky` instead of `position: fixed`,
as it allows to avoid padding hacks.

Also remove manual Viewport.doLayout() hacks, as we can
now use `width: auto` as replacement for a static
snapped-module menu width.

Resolves: #93353
Releases: master
Change-Id: I7d56a3b3b405e36d65c9a87a03360074354c3d9d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/67502


Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>