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

Currently, authorizations/new view has a fixed hidden_field_tags.
This is 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 easyly pass the new parameter and can avoid repeating in codes.

Use 2 spaces for indentation in a hash, relative to the start of the line where the left curly brace is.