match font family when editing heading cells

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

@@ -40,7 +40,10 b' div.cell.text_cell.rendered {'
 .cm-s-heading-3,
 .cm-s-heading-4,
 .cm-s-heading-5,
-.cm-s-heading-6 { font-weight: bold;}
+.cm-s-heading-6 {
+    font-weight: bold;
+    font-family: @font-family-sans-serif;
+}
 
 .cm-s-heading-1 { font-size:150%; }
 .cm-s-heading-2 { font-size: 130%; }

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

@@ -1133,6 +1133,7 b' div.cell.text_cell.rendered {'
 .cm-s-heading-5,
 .cm-s-heading-6 {
   font-weight: bold;
+  font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
 }
 .cm-s-heading-1 {
   font-size: 150%;

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

@@ -8757,6 +8757,7 b' div.cell.text_cell.rendered {'
 .cm-s-heading-5,
 .cm-s-heading-6 {
   font-weight: bold;
+  font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
 }
 .cm-s-heading-1 {
   font-size: 150%;