Unused ember-test-selectors dependency

[guidojw]

With merging #398, I noticed that we aren't really using this dependency at the moment. There are quite a few data-test-* attributes set in the templates but the tests don't reference these. They were probably used in the acceptance tests we used to have, which were removed in a Ember.js update a while ago.

My proposal is to remove the dependency and attributes and reinstall/reconfigure it again when we need it someday.

[guidojw]

#539 starts using it again