Propagate venue deletions to BeerPrice instances (#232)

That way when a venue closes, we don't have to manually delete
pricing info.

Fixes #231

beers/migrations/0030_auto_20190807_2142.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2.4 on 2019-08-07 21:42
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('beers', '0029_merge_20190519_1259'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='beerprice',
+            name='venue',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='beer_prices', to='venues.Venue'),
+        ),
+    ]

beers/models.py

@@ -273,7 +273,7 @@ def __str__(self):
 class BeerPrice(models.Model):
     beer = models.ForeignKey(Beer, models.CASCADE, related_name='prices')
     venue = models.ForeignKey(
-        'venues.Venue', models.DO_NOTHING, related_name='beer_prices',
+        'venues.Venue', models.CASCADE, related_name='beer_prices',
     )
     serving_size = models.ForeignKey(
         ServingSize, models.DO_NOTHING, related_name='beer_prices',