Revert 2em notebook header space

IPython/html/static/notebook/less/notebook.less

@@ -35,7 +35,7 b' div#notebook {'
     overflow-x: auto;
     width: 100%;
     /* This spaces the cell away from the edge of the notebook area */
-    padding: 2em 0 1em 0;
+    padding: 1em 0 1em 0;
     margin: 0px;
     border-top: 1px solid @navbar-default-border;
     outline: none;