fix(setup): Use new pre/post profile handlers

[Per
feedback](#66 (comment)), use
the new (to me) `post_handler` feature provided by `GenericSetup` as it is much better
than littering the import step registry with one-off, profile-specific import steps.

src/Products/PlonePAS/profiles.zcml

@@ -33,14 +33,6 @@
       description="Change the Zope root `/acl_users` to use a simple cookie login form
 		   instead of HTTP `Basic ...` for authentication."
       provides="Products.GenericSetup.interfaces.EXTENSION"
-      />
-  <genericsetup:importStep
-      name="zope-root-auth-cookie"
-      title="Zope Root Cookie Login"
-      description="Change the Zope root `/acl_users` to use a simple cookie login form
-		   instead of HTTP `Basic ...` for authentication."
-      handler=".setuphandlers.set_up_zope_root_cookie_auth">
-    <depends name="plonepas" />
-  </genericsetup:importStep>
+      post_handler=".setuphandlers.set_up_zope_root_cookie_auth" />
 
 </configure>

src/Products/PlonePAS/setuphandlers.py

@@ -523,10 +523,7 @@ def set_up_zope_root_cookie_auth(context):
     """
     Change the Zope root `/acl_users` to use a simple cookie login form.
     """
-    # Only run step if a flag file is present, IOW not for every profile
-    if context.readDataFile("plone-pas-zope-root-cookie.txt") is None:
-        return
-    portal = context.getSite()
+    portal = aq_parent(context)
     root = portal.getPhysicalRoot()
     root_acl_users = getToolByName(root, "acl_users")