Every campaign has an end.

Drop ahoy, because it's more trouble than it's worth. This means dropping
visits, campaigns, targets.

Campaigns are better run to analytics, such as:
* https://matomo.org/docs/tracking-campaigns/
* https://matomo.org/docs/tracking-goals-web-analytics/
* https://support.google.com/analytics/answer/1012040?hl=en

db/migrate/20181009000259_drop_ahoy_events.rb

@@ -0,0 +1,15 @@
+class DropAhoyEvents < ActiveRecord::Migration[5.0]
+  def up
+    remove_index :visits, [:visit_token]
+    remove_index :ahoy_events, [:name, :time]
+
+    drop_table :targets
+    drop_table :campaigns
+    drop_table :visits
+    drop_table :ahoy_events
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end