Move config into meta.

* Adds `{{CONFIG_META}}` to the `app/index.html` and `tests/index.html`.
* Adds module at `<my-app-name>/config/environment` that simply reads the current meta value.
* Makes build output the same regardless of environment config.
* Makes injection of custom config information as simple as adding/modifying/customizing the meta contents.

Closes #2059.