Refactor / rename Billing object classes (#1993)

This includes renaming the billing classes to match the SQL table names,
as well as splitting them out into their own separate top-level classes.
The rest of the changes are mostly renaming variables and comments etc.

We now use `BillingBase` as the name of the common billing superclass,
because one-time events are called BillingEvents

db/src/main/resources/sql/schema/db-schema.sql.generated

@@ -41,9 +41,9 @@
         flags text[],
         reason text not null,
         domain_name text not null,
-        billing_time timestamptz,
         billing_event_id int8,
         billing_recurrence_id int8,
+        billing_time timestamptz,
         primary key (billing_cancellation_id)
     );