Add timestamp columns to event_types

... required for Rails to calculate cache keys

db/migrate/20180226032958_add_created_at_and_updated_at_to_event_types.rb

@@ -0,0 +1,12 @@
+class AddCreatedAtAndUpdatedAtToEventTypes < ActiveRecord::Migration[5.0]
+  def up
+    add_column :event_types, :created_at, :datetime
+    add_column :event_types, :updated_at, :datetime
+    EventType.update_all(created_at: Time.now, updated_at: Time.now)
+  end
+
+  def down
+    remove_column :event_types, :created_at
+    remove_column :event_types, :updated_at
+  end
+end