restore anchore on h2-h6 links

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

@@ -30,6 +30,8 b' a.anchor-link:link {'
   visibility: hidden;
 }
 
-h1:hover .anchor-link {
-  visibility: visible;
-}
  No newline at end of file
+h1,h2,h3,h4,h5,h6 {
+    &:hover .anchor-link {
+        visibility: visible;
+    }
+}

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

@@ -1040,7 +1040,7 b' div.text_cell_input{color:#000000;border:1px solid #cfcfcf;border-radius:4px;bac'
 div.text_cell_render{outline:none;resize:none;width:inherit;border-style:none;padding:5px;color:#000000;}
 a.heading-anchor{text-decoration:none;color:inherit;}
 a.anchor-link:link{text-decoration:none;padding:0px 20px;visibility:hidden;}
-h1:hover .anchor-link{visibility:visible;}
+h1:hover .anchor-link,h2:hover .anchor-link,h3:hover .anchor-link,h4:hover .anchor-link,h5:hover .anchor-link,h6:hover .anchor-link{visibility:visible;}
 .toolbar{padding:3px 15px;border-bottom:1px #ababab solid;}.toolbar button{margin-top:2px;margin-bottom:2px;}
 .toolbar select,.toolbar label{height:19px;vertical-align:middle;margin-right:2px;margin-bottom:0;display:inline;font-size:92%;margin-left:0.3em;margin-right:0.3em;padding:0px;}
 .toolbar select{width:auto;}