Fetch security settings from the registry instead of portal properties

All security settings are now looked up from the plone registry instead of portal properties. This work is related to https://github.com/plone/Products.CMFPlone/issues/216 .

Additionally, some of the security related tests from plone.app.controlpanel [1] were converted to unit/integration tests and moved to this package, since they were basically testing the ``@@new-user`` form with generate_user_id and generate_login_name methods, based on the security settings in the registry. So I think the various cases are better tested here, while the more general browser tests for the security panel will be implemented as robot tests in CMFPlone, where the security panel will be moved.

This PR also contains a fix for https://github.com/plone/Products.CMFPlone/issues/261 , discovered when adding additional tests.

[1] https://github.com/plone/plone.app.controlpanel/blob/master/plone/app/controlpanel/tests/security.txt