Week7 part2

Mostly refactoring with decorator and service object
Oh, I did a trick to allow me still use `flash[:error]`.

In [shared/_messages.html.haml] (https://github.com/ascendbruce/myflix/blob/master/app/views/shared/_messages.html.haml#L2) partial, it pass flash.key to [alert_class](https://github.com/ascendbruce/myflix/blob/master/app/helpers/application_helper.rb#L3) helper and get proper classes back.

    # app/helpers/application_helper.rb
    bootstrap_name = { error: "danger" }[alert_type.to_sym] || "info"
    # ...