fix more style consistencies

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

@@ -49,5 +49,11 b' div.cell.text_cell.rendered {'
 .cm-s-heading-2 { font-size: 130%; }
 .cm-s-heading-3 { font-size: 120%; }
 .cm-s-heading-4 { font-size: 110%; }
-.cm-s-heading-5 { font-size: 100%; }
-.cm-s-heading-6 { font-size: 90%; }
+.cm-s-heading-5 {
+    font-size: 100%; 
+    font-style: italic;
+}
+.cm-s-heading-6 {
+    font-size: 90%; 
+    font-style: italic;
+}

IPython/html/static/style/ipython.min.css

@@ -1149,9 +1149,11 b' div.cell.text_cell.rendered {'
 }
 .cm-s-heading-5 {
   font-size: 100%;
+  font-style: italic;
 }
 .cm-s-heading-6 {
   font-size: 90%;
+  font-style: italic;
 }
 .widget-area {
   /*

IPython/html/static/style/style.min.css

@@ -8773,9 +8773,11 b' div.cell.text_cell.rendered {'
 }
 .cm-s-heading-5 {
   font-size: 100%;
+  font-style: italic;
 }
 .cm-s-heading-6 {
   font-size: 90%;
+  font-style: italic;
 }
 .widget-area {
   /*