Add ability to disable user code on page load

Adding `run=disabled` to the uri will disable the
page from running user code.

This is useful for pages that have frozen to to infinite loops
or untrusted links that may be malicious

client/commonFramework/update-preview.js

@@ -74,7 +74,7 @@ window.common = (function(global) {
         preview.write(
           libraryIncludes +
           jQuery +
-          code +
+          (common.codeUri.shouldRun() ? code : '' ) +
           '<!-- -->' +
           iframeScript
         );