Part I: User Creation

1. add route for new_user_path
2. create a UsersController with new action
3. create new.html.erb
4. generate user model with password_digest string field
5. uncomment gem 'bcrypt' in Gemfile and add has_secure_password in User model
6. add create action in UsersController
7. implement logic for creating a user
8. set session[:user_id] in create action
9. add route for user_path (the show action)
10. addshow action in UsersController
11. add show.html.erb
12. create current_user helper method in ApplicationController

Part II: Logging in

1. add login route
2. create SessionsController
3. add new action in SessionsController
4. add new.html.erb
5. make login form in new.html.erb
6. add post route for login
7. add create action in SessionsController
8. implement create action: find user, authenticate user, set session[:user_id], and redirect

Part III: Logging Out

1. add logout link
2. add logout route
3. add destroy action in SessionsController
4. clear the session and redirect