Resolve collapsing margin between `.layout-page` and `.content-wrapper-margin`
Description
(Identified in !63254 (comment 593232261))
There's a number of things which should cause a page-wide top margin (i.e. performance bar, or system header, or both). In this MR !63254 (merged) we've decoupled the margin from our content-wrapper class since the content-wrapper is already coupled to a number of other things. But! Some of this top margin is actually defined and handled by .layout-page
Proposal
Let's have a single class that is responsible for this page-wide top margin (and also bottom margin if there's a system footer).
Implementation notes
It seems that .layout-page is used to wrap the sidebar and the .content-wrapper:
Maybe we just want to move .content-wrapper-margin over to .layout-page (along with the .layout-page specific margin rules)?