Add users action to edit their profile

- Add UsersController#edit
- Add UsersController#update
Add line to save the token in sign_in helper

This is because filling the form doesn't work when not using capybara
Add settings link to the header

This just works if the user is already authenticated
Add spec for valid user editing
Authentication implementation

With that, the friendly forwarding integration test in Listing 9.17 should pass,
and the basic user authentication and page protection implementation is complete.
Kept working on the authorization mechanism
Add pagination with will paginate

- Add specs for testing the pagination
Refactor: Add a partial for users listing
Add admin attribute for user model
Finish user edit, update, index and destroy actions