articleLayout.container.flexDirection='column' only when media is less than 'small' (#1416)

* Specify articleLayout.container to flex in column direction

* articleLayout.container.flexDirection=column when media less than small

* Comply with eslint rules

src/theme.js

@@ -121,6 +121,9 @@ const sharedStyles = {
         marginLeft: 'auto',
         marginRight: 'auto',
       },
+      [media.lessThan('small')]: {
+        flexDirection: 'column',
+      },
     },
     content: {
       marginTop: 40,