Bug 1042887 - Cannot scroll in messages app  (fixed by moving "overflow:auto" up one level, to the flex item)

See https://bugzilla.mozilla.org/show_bug.cgi?id=1042887#c15 -- this fixes that bug by moving "overflow:auto" from a height:100% element up to its parent (which is a flex item), to prevent the flex item from having too large of an implied minimum size.