title: Devise: sign in with two possible encrypted passwords
body: <p>My app works with the Devise gem for authentication, but I want customize it for sign in with two possible encrypted passwords, due to my previous app worked with MD5. I've two fields in my users table: <strong>encrypted_password</strong> and <strong>encrypted_old_password</strong> (I've created), I want check if exists the value <strong>encrypted_password</strong> and if the password sent matches the one set, otherwise, check if it does with MD5 and if true, then replace the value <strong>encrypted_password</strong>.</p>
  
  <p>How I do this?</p>