Fixing environment detection in App.Router

To make the web acceptance tests function as expected when hitting `/app_test.php` we had a hack in the `base.html.twig` file that set an `appRoot` JS variable based off the environment name.

This PR removes that hack, and adds a cleaner solution by setting a cookie containing the environment name, and then having `App.Router` determine what the base root is from the environment.

Fixes #17.