Add a CSS component for a page's main content box.
Review Request #10787 — Created Nov. 5, 2019 and submitted
.rb-c-page-content-boxcomponent is used to display a main
content area that contains a border and padding in a desktop display but
none in a mobile display. This is a pretty standard appearance for pages
like datagrids and in the administration UI, one which has been
re-implemented in a few different places, or has been built upon the old
Currently this component isn't being used anywhere, but it's being used
in much of the new admin UI.
Tested with some upcoming admin UI work, and verified the appearance was
correct in both desktop and mobile modes.