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- Removes mdtest which is no longer valid (to be replaced with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/models/model-test.js.es6

@@ -18,4 +18,4 @@ QUnit.test("extractByKey: converts a list of hashes into a hash of instances of
 QUnit.test("extractByKey: returns an empty hash if there isn't anything to convert", assert => {
   assert.deepEqual(Model.extractByKey(), {}, "when called without parameters");
   assert.deepEqual(Model.extractByKey([]), {}, "when called with an empty array");
-});
+});