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.

`end` at 13, 51 is not aligned with `let(:client) { double :client, redirect_uri: 'http://tst.com/auth',` at 12, 4