Use HTMLBars from published NPM package.

* Removes need to do crazy monkey patching/nesting inside HTMLBars.
* Enables us to use the specific version of simple-html-tokenizer for
  the version of HTMLBars we are using.
* Do not use `htmlbars-runtime` (ember-htmlbars is our runtime).
* Use `htmlbars-test-helpers` package instead of grabbing
  `test/support/`.