The new .rb-c-page-content-box component 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

.box classes.
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.