Make the static elements tracked users only

We cannot track anonymous users as the session expires after 10 minutes of
inactivity. This also avoids an issue where there is an error when the user's
session has expired and they navigate a page

src/wagtail_personalisation/models.py

@@ -1,6 +1,7 @@
 from __future__ import absolute_import, unicode_literals
 
 from django import forms
+from django.conf import settings
 from django.contrib.sessions.models import Session
 from django.db import models, transaction
 from django.template.defaultfilters import slugify
@@ -82,7 +83,9 @@ class Segment(ClusterableModel):
             "set until the number is reached. After this no more users will be added."
         )
     )
-    sessions = models.ManyToManyField(Session)
+    static_users = models.ManyToManyField(
+        settings.AUTH_USER_MODEL,
+    )
 
     objects = SegmentQuerySet.as_manager()
 
@@ -131,7 +134,7 @@ class Segment(ClusterableModel):
 
     @property
     def is_full(self):
-        return self.sessions.count() >= self.count
+        return self.static_users.count() >= self.count
 
     def encoded_name(self):
         """Return a string with a slug for the segment."""