Feature: Change markdown engine to markdown it

This commit removes the old evilstreak markdownjs engine.

- Adds specs to WhiteLister and changes it to stop using globals
    (Fixes large memory leak)
- Fixes edge cases around bbcode handling
- Removes mdtest which is no longer valid (to be replaced with
    CommonMark)
- Updates MiniRacer to correct minor unmanaged memory leak
- Fixes plugin specs

plugins/discourse-details/assets/javascripts/lib/discourse-markdown/details.js.es6

@@ -1,25 +1,3 @@
-import { registerOption } from 'pretty-text/pretty-text';
-
-function insertDetails(_, summary, details) {
-  return `<details><summary>${summary}</summary>${details}</details>`;
-}
-
-// replace all [details] BBCode with HTML 5.1 equivalent
-function replaceDetails(text) {
-  text = text || "";
-
-  while (text !== (text = text.replace(/\[details=([^\]]+)\]((?:(?!\[details=[^\]]+\]|\[\/details\])[\S\s])*)\[\/details\]/ig, insertDetails)));
-
-  // add new lines to make sure we *always* have a <p> element after </summary> and around </details>
-  // otherwise we can't hide the content since we can't target text nodes via CSS
-  return text.replace(/<\/summary>/ig, "</summary>\n\n")
-             .replace(/<\/details>/ig, "\n\n</details>\n\n");
-}
-
-registerOption((siteSettings, opts) => {
-  opts.features.details = true;
-});
-
 const rule = {
   tag: 'details',
   before: function(state, attrs) {
@@ -46,11 +24,7 @@ export function setup(helper) {
     'details.elided'
   ]);
 
-  if (helper.markdownIt) {
-    helper.registerPlugin(md => {
-      md.block.bbcode_ruler.push('details', rule);
-    });
-  } else {
-    helper.addPreProcessor(text => replaceDetails(text));
-  }
+  helper.registerPlugin(md => {
+    md.block.bbcode_ruler.push('details', rule);
+  });
 }