put htmlmixed.js and css.js includes in CMS base.html so that we can support html mode in CodeMirror

cms/templates/base.html

@@ -33,6 +33,8 @@
     <script src="${static.url('js/vendor/jquery.leanModal.min.js')}"></script>
     <script src="${static.url('js/vendor/jquery.tablednd.js')}"></script>
     <script src="${static.url('js/vendor/jquery.form.js')}"></script>
+    <script type="text/javascript" src="${static.url('js/vendor/CodeMirror/htmlmixed.js')}"></script>
+    <script type="text/javascript" src="${static.url('js/vendor/CodeMirror/css.js')}"></script>
     <script type="text/javascript">
       document.write('\x3Cscript type="text/javascript" src="' +
           document.location.protocol + '//www.youtube.com/player_api">\x3C/script>');

common/lib/xmodule/xmodule/js/src/html/edit.coffee

@@ -1,6 +1,7 @@
 class @HTMLEditingDescriptor
   constructor: (@element) ->
     @edit_box = CodeMirror.fromTextArea($(".edit-box", @element)[0], {
+      mode: "text/html"
       lineNumbers: true
       lineWrapping: true
     })