Redesign: move RightPanel below RoomHeader

[bwindels]

• move mx_RoomView_auxPanel into mx_RoomView_body
• move right panel header into group/room header
• create MainSplit component to use in RoomView and GroupView
• remove RightPanel from LoggedInView & pass collapsedRhs to RoomView/GroupView
• put MainSplit around mx_GroupView_body
• put MainSplit around mx_RoomView_body