Change user workflows to prevent user enumeration attacks

[andreslucena]

🎩 What? Why?

On some workflows, it's possible to check if an email was registered in the application. For instance, in the "Forgot your password?" page if an attacker tries with a registered email, she'll get an error message (You will receive an email with instructions on how to reset your password in a few minutes.) that's different to a non-existing user account ("could not be found. Did you sign up previously?"). This attack gets the name of "Account enumeration".

This PR fixes this partially, as it isn't solved in registration form, but it is solved in others like "forgot your password" or "resend unlock instructions".

Testing

1. Go to Resend confirmation instructions, Forgot your password?, or Resend unlock instructions forms
2. Enter an existing user email
3. Click in "Send button"
4. See alert
5. Go back to the same form
6. Enter a non-existing user email
7. Click in "Send button"
8. See that you have the same alert

📷 Screenshots

♥️ Thank you!