Implemented get_related_model in utils.compat

wagtail/utils/compat.py

@@ -0,0 +1,5 @@
+def get_related_model(rel):
+    # In Django 1.7 and under, the related model is accessed by doing: rel.model
+    # This was renamed in Django 1.8 to rel.related_model. rel.model now returns
+    # the base model.
+    return getattr(rel, 'related_model', rel.model)

wagtail/wagtailadmin/edit_handlers.py

@@ -6,7 +6,6 @@ import warnings
 from six import text_type
 
 from modelcluster.forms import ClusterForm, ClusterFormMetaclass
-from modelcluster.models import get_related_model
 
 from django.db import models
 from django.template.loader import render_to_string
@@ -23,6 +22,7 @@ from wagtail.wagtailadmin import widgets
 from wagtail.wagtailcore.models import Page
 from wagtail.wagtailcore.utils import camelcase_to_underscore, resolve_model_string
 from wagtail.utils.deprecation import RemovedInWagtail12Warning
+from wagtail.utils.compat import get_related_model
 
 
 # Form field properties to override whenever we encounter a model field

wagtail/wagtailadmin/utils.py

@@ -5,10 +5,10 @@ from django.contrib.auth import get_user_model
 from django.db.models import Q
 
 from modelcluster.fields import ParentalKey
-from modelcluster.models import get_related_model
 
 from wagtail.wagtailcore.models import Page, PageRevision, GroupPagePermission
 from wagtail.wagtailusers.models import UserProfile
+from wagtail.utils.compat import get_related_model
 
 
 def get_object_usage(obj):