Feature: CommonMark support

This adds the markdown.it engine to Discourse.
https://github.com/markdown-it/markdown-it

As the migration is going to take a while the new engine is default
disabled. To enable it you must change the hidden site setting:
enable_experimental_markdown_it.

This commit is a squash of many other commits, it also includes some
improvements to autospec (ability to run plugins), and a dev dependency
on the og gem for html normalization.

test/javascripts/components/d-editor-test.js.es6

@@ -21,7 +21,7 @@ componentTest('preview sanitizes HTML', {
   template: '{{d-editor value=value}}',
 
   test(assert) {
-    this.set('value', `"><svg onload="prompt(/xss/)"></svg>`);
+    fillIn('.d-editor-input', `"><svg onload="prompt(/xss/)"></svg>`);
     andThen(() => {
       assert.equal(this.$('.d-editor-preview').html().trim(), '<p>\"&gt;</p>');
     });