Fix missing scrollbar in left pane when too many collections/notebooks (#375)

Constrain left pane container to height: 100% so that scrollbar show up when content wants to overflow.
The `main` classname seems too generic, but I left it alone (so I don't break anything), since this part will eventually be ported to React.

less/tree.less

@@ -4,6 +4,9 @@
 .resourceTree {
     height: 100%;
     flex: 0 0 auto;
+    .main {
+        height: 100%;
+    }
 }
 
 .resourceTreeScroll {