frontend: Remove extra margin when res between sm and md

We allow the sidebar to be opened on any width other than sm, but the
main component was adding an extra margin between that measure and md
which resulted in an extra and useless blank space.

These changes add that margin only on sm.

frontend/src/components/App/Layout.tsx

@@ -14,7 +14,7 @@ const useStyle = makeStyles(theme => ({
   content: {
     flexGrow: 1,
     marginLeft: 'initial',
-    [theme.breakpoints.between('sm', 'md')]: {
+    [theme.breakpoints.only('sm')]: {
       marginLeft: drawerWidthClosed,
     },
   },