Rework views

* Adds a hash field which is filled before storing the object to the database
* Adds a view `view_user`
* Redirects to that view after storing the object to the database
* Adds a view `confirmation` which shows the confirmation page and takes care of persisting to the database.
* Adds the option `show_confirmation` (default: `True`) to `register_schema()`. If set to `False`, the confirmation page will not be shown.
* Allows to use different form templates for the different views.

The mapping between view and form template is as follows:

View `form`: `[templates_user/]form.pt`
View `confirmation`: `[templates_user/]readonly/form_confirmation.pt`
View `view_user`: `[templates_user/]readonly/form_view_user.pt`
View `edit` (admin): `[templates_admin/]form.pt`
View `view_admin` (admin): `[templates_admin/]readonly/form.pt`

Closes #61, Closes #58, Closes #51