The sidebar currently uses mw-ui-icon so we continue this
practice, however we provide a general rule to ensure all icons
rendered through it default to 20x20. This didn't impact the side
bar icon as that already specifies a height of 20px.
Bug: T268241
Change-Id: I6f8e8400da048a97cbf59c3e6ad918763fc91041
jdlrobson