Use generate_token_for instead of dedicated model for passwordless

[driveton]

The gem currently uses a dedicated model for sign_in tokens which seems not that needed while we can use generate_token_for to allow users logging in without a password similar to password resets.

user.rb

generates_token_for :passwordless_sign_in, expires_in: 1.day do
    "#{email}-#{updated_at.to_i}"
end 

passwordlesses_controller.rb

def revoke_tokens
      @user.touch
end

• Invalidates the token after a day (same to the current behavior)
• Invalidates the token if the user updates their password
• Invalidates the token if the user signs in with the token
• We remove the need to track tokens in a model