Sending new and existing students emails when they are added to a class. 

Students receive email when they are added to a class. If the student is new to Quizroom, they are taken to complete_signup_path where they can update their account info and are then redirected to the classroom_path. 

I wrote an integration spec for StudentsController#create (with the help of Mike G.) and also refactored the add_students feature spec. 

I also added the database cleaner gem. Turns out that when running the entire test suite, some integration specs were failing that shouldn't have been. When Selenium ran in some specs, it created objects in the test db that weren't getting deleted. That's been fixed.