Display sidebar scroll-bar only if required

This fix a regression from #316 (scrollbar was displayed all the time).

Note that the padding-bottom value is totally empiric, but proved OK on my Fx
and Chrome instances + some responsive tests. There might be finer solutions,
feel free :-).

ihatemoney/static/css/main.css

@@ -74,12 +74,13 @@ body {
     background-repeat: no-repeat;
     height: 100%;
     color: black;
-    overflow-y: scroll;
+    overflow-y: auto;
 }
 
 @media (min-width: 768px) {
     .sidebar {
         position: fixed;
+        padding-bottom: 4.5rem;
     }
 }