Fix Revision model change in Wagtail 4.0 (#630)

this should cover:
- adding localize to a Wagtail 4 project
- upgrading a project using localize to Wagtail 4

wagtail_localize/migrations/0001_initial.py

@@ -1,20 +1,35 @@
 # Generated by Django 3.0.6 on 2020-07-17 15:07
 
-from django.db import migrations, models
 import django.db.models.deletion
 
+from django.db import migrations, models
+from django.db.migrations.recorder import MigrationRecorder
+from wagtail import VERSION as WAGTAIL_VERSION
+
 
-class Migration(migrations.Migration):
+def get_revision_model():
+    revision_model = "wagtailcore.PageRevision"
+    if WAGTAIL_VERSION >= (4, 0, 0):
+        try:
+            if MigrationRecorder.Migration.objects.filter(
+                app="wagtailcore", name="0070_rename_pagerevision_revision"
+            ).exists():
+                revision_model = "wagtailcore.Revision"
+        except (django.db.utils.OperationalError, django.db.utils.ProgrammingError):
+            # normally happens when running tests
+            pass
 
+    return revision_model
+
+
+class Migration(migrations.Migration):
     initial = True
 
     dependencies = [
         ("wagtailcore", "0057_page_locale_fields_notnull"),
         ("contenttypes", "0002_remove_content_type_name"),
     ]
 
-    run_before = [("wagtailcore", "0059_apply_collection_ordering")]
-
     operations = [
         migrations.CreateModel(
             name="String",
@@ -149,7 +164,7 @@ class Migration(migrations.Migration):
                         null=True,
                         on_delete=django.db.models.deletion.SET_NULL,
                         related_name="+",
-                        to="wagtailcore.PageRevision",
+                        to=get_revision_model(),
                     ),
                 ),
                 (