Use GitHub account to login

1. login and logout successfully on localhost
2. add a dropdown menu for logged in user
3. move current_user function to session_help.rb

issue: After deployed on Heroku, https://grisly-grave-6273.herokuapp.com, redirecting after signed in still throws 404 error  (already registered a new github application). No such problem on localhost. :(((((((
Use 2 (not 1) spaces for indentation.<br>Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.