Revert "Change stretch to flex-start"

This reverts commit 774073116dd4195037c452d48f05b432bba38801.

IPython/html/static/base/less/flexbox.less

@@ -35,7 +35,7 b' Browsers not listed, including Safari, are supported via the styling under the'
     /* Modern browsers */
     display: flex;
     flex-direction: row;
-    align-items: flex-start;
+    align-items: stretch;
 }
 
 .hbox > * {
@@ -67,7 +67,7 b' Browsers not listed, including Safari, are supported via the styling under the'
     /* Modern browsers */
     display: flex;
     flex-direction: column;
-    align-items: flex-start;
+    align-items: stretch;
 }
 
 .vbox > * {