support multiple uses of the mixin class (#140)

src/wagtail_personalisation/models.py

@@ -103,7 +103,7 @@ class PersonalisablePageMixin(models.Model):
         blank=True, null=True
     )
     segment = models.ForeignKey(
-        Segment, related_name='segments', on_delete=models.PROTECT,
+        Segment, related_name='+', on_delete=models.PROTECT,
         blank=True, null=True
     )
     is_segmented = models.BooleanField(default=False)