Deleted date should be null-able

Fixes #240

bookwyrm/migrations/0056_auto_20201021_0150.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.0.7 on 2020-10-21 01:50
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('bookwyrm', '0055_merge_20201017_0011'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='status',
+            name='deleted_date',
+            field=models.DateTimeField(blank=True, null=True),
+        ),
+    ]