Refactor personalisable pages

Instead of working with django model mixins it now uses a separate
model entity to keep track of the personalized pages (metadata).

The current downside of this approach is that the segment of an existing
variant is no longer easily adjustable for now.

src/wagtail_personalisation/views.py

@@ -122,11 +122,13 @@ def copy_page_view(request, page_id, segment_id):
     if request.user.has_perm('wagtailadmin.access_admin'):
         segment = get_object_or_404(Segment, pk=segment_id)
         page = get_object_or_404(Page, pk=page_id).specific
-        variants = page.variants_for_segments([segment])
+
+        metadata = page.personalisable_metadata
+        variants = metadata.variants_for_segments([segment])
         if variants.exists():
             variant = variants.first()
         else:
-            variant = page.copy_for_segment(segment)
+            variant = metadata.copy_for_segment(segment)
         edit_url = reverse('wagtailadmin_pages:edit', args=[variant.id])
 
         return HttpResponseRedirect(edit_url)