Moves activitypub mixin to its own file

bookwyrm/migrations/0017_auto_20201130_1819.py

@@ -1,6 +1,6 @@
 # Generated by Django 3.0.7 on 2020-11-30 18:19
 
-import bookwyrm.models.base_model
+import bookwyrm.models.activitypub_mixin
 import bookwyrm.models.fields
 from django.conf import settings
 from django.db import migrations, models
@@ -38,7 +38,7 @@ class Migration(migrations.Migration):
             options={
                 'abstract': False,
             },
-            bases=(bookwyrm.models.base_model.ActivitypubMixin, models.Model),
+            bases=(bookwyrm.models.activitypub_mixin.ActivitypubMixin, models.Model),
         ),
         migrations.AddField(
             model_name='user',