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- Removes mdtest which is no longer valid (to be replaced with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

lib/pretty_text/shims.js

@@ -7,8 +7,6 @@ __utils = require('discourse/lib/utilities');
 __emojiUnicodeReplacer = null;
 
 __setUnicode = function(replacements) {
-  require('pretty-text/engines/discourse-markdown/emoji').setUnicodeReplacements(replacements);
-
   let unicodeRegexp = new RegExp(Object.keys(replacements).sort().reverse().join("|"), "g");
 
   __emojiUnicodeReplacer = function(text) {