Replace ember-auth with ember-simple-auth

Well, the short version of why I needed to do this change:

The [ember-auth](https://github.com/heartsentwined/ember-auth) that we were using has a lot o bugs and it is not getting updates since oct last year. The project is not prepared for testing environment and it was causing a lot of instability to our test suite. Also it was getting in my way to develop new features.