Fix passwordless_sign_in bugs

[yshmarov]

failing tests at passwordless_sign_in(user) led me on such a trail....

• @token was not being set correctly
• we were using id instead of the identifier
• minor improvements in the html template (email_field_tag provides basic validations)

[mikker]

Ugh no, that's a bug!

Can we fix the tests so they would've caught this?

[yshmarov]

without these changes, error can be reproduced like this:

    session = Passwordless::Session.create!(authenticatable: user)
    email = Passwordless::Mailer.sign_in(session)
    assert_raises(ActionController::UrlGenerationError) do
      email.to
    end

[mikker]

❤️

[mikker]

Thank you once again ❤️🧡💛💚💙💜