Move hidden fields from view to pre_authorization, for supporting other parameters.

Currently, authorizations/new view has hardwired `hidden_field_tags`, used to pass parameters, such as scope and state, between authN and authZ.

I also trying to support OpenID Connect on this Doorkeeper, so I want to pass a new parameter `nonce`.
Using my changes, we can easily pass the new parameter and can avoid overriding/repeating code.