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- Removes mdtest which is no longer valid (to be replaced with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

test/javascripts/test_helper.js

@@ -143,6 +143,5 @@ Object.keys(requirejs.entries).forEach(function(entry) {
     require(entry, null, null, true);
   }
 });
-require('mdtest/mdtest', null, null, true);
 resetSite();