Add underline button to CommonMark Markdown toolbar (#40939).

Contributed by Yasu Saku (user:skys).


git-svn-id: https://svn.redmine.org/redmine/trunk@23106 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/assets/javascripts/jstoolbar/common_mark.js

@@ -26,6 +26,16 @@ jsToolBar.prototype.elements.em = {
   }
 }
 
+// u
+jsToolBar.prototype.elements.ins = {
+  type: 'button',
+  title: 'Underline',
+  shortcut: 'u',
+  fn: {
+    wiki: function() { this.singleTag('<u>', '</u>') }
+  }
+}
+
 // del
 jsToolBar.prototype.elements.del = {
   type: 'button',