Ensure that all lines in the pad are marked with class `ace-line`.

Without this change, lines that haven't ever been edited will have either
an empty class or, in the case of list start lines, a class that begins
with a space (because the `ace-line` before the space never got added).

src/static/js/domline.js

@@ -214,7 +214,7 @@ domline.createDomLine = function(nonEmpty, doesWrap, optBrowser, optDocument)
   result.clearSpans = function()
   {
     html = [];
-    lineClass = ''; // non-null to cause update
+    lineClass = 'ace-line';
     result.lineMarker = 0;
   };