Allow more fields to be optional in custom Page admins.

See issue #536. When using a custom Page model and corresponding
admin class and still using the original PageAdminForm, you couldn't
disable the redirect_to and override_url (and less important: active)
fields because the form did not check first if they were in the
fieldset or not.
Merge pull request #537 from Gwildor/master

Allow more fields to be optional in custom Page admins.
Merge branch 'master' into next

* master:
  Allow more fields to be optional in custom Page admins.