Fix notes on Step 12 to reference the correct method

Step 12 adds the `authenticate_user!` method to the `users_controller` but the notes reference `redirect_if_authenticated`.

Fix notes on Step 12 to reference the correct method

Step 12 adds the `authenticate_user!` method to the `users_controller` but the notes reference `redirect_if_authenticated`.

Update mailer previews

Add correct setup data to make mailers work.

Issues
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- Closes #65

Proof README.

Update application.html.erb

Tags header and main were not closed

Add status: :unprocessable_entity to all failed render responses

This will help keep things consistent with Rails 7 and Turbo.

Issues
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- Closes #52

Create setup script.

Issues
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- Closes #42