Refactor request_phase function.

The request params are already accessible in request.params so there is no
need to get them from the session.

There is also no need to conditionally initialize options[:authorize_params]
and options[:request_params] as these are already set automatically by
omniauth-oauth.
https://github.com/intridea/omniauth-oauth/blob/43008e42b257d9b7c0b98122cf03ce14b021d782/lib/omniauth/strategies/oauth.rb#L16

This refactor will make it really easy to add new params in the future.
Add i18n support.

Pass the `lang` param to the auth request to dynamically set the language
of the Twitter prompt:
/auth/twitter?lang=pt

The language of the prompt could already be initially set in the OmniAuth
config with:
:authorize_params => { :lang => 'pt' }

Fix #45