Add migrations for making ExtensibleModel the default base

Advances #182

aleksis/core/migrations/0013_extensible_model_as_default.py

+# Generated by Django 3.0.3 on 2020-02-20 12:24
+
+import aleksis.core.util.core_helpers
+import django.contrib.postgres.fields.jsonb
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0012_announcement'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='activity',
+            name='created_at',
+        ),
+        migrations.RemoveField(
+            model_name='notification',
+            name='created_at',
+        ),
+        migrations.AddField(
+            model_name='activity',
+            name='extended_data',
+            field=django.contrib.postgres.fields.jsonb.JSONField(default=dict, editable=False),
+        ),
+        migrations.AddField(
+            model_name='announcement',
+            name='extended_data',
+            field=django.contrib.postgres.fields.jsonb.JSONField(default=dict, editable=False),
+        ),
+        migrations.AddField(
+            model_name='notification',
+            name='extended_data',
+            field=django.contrib.postgres.fields.jsonb.JSONField(default=dict, editable=False),
+        ),
+    ]