Add a converter for CurrencyUnits stored in the database (#334)

* Add a converter for CurrencyUnits stored in the database

This uses the well-known String representation for currency units. It also
provides a base class for other converters that will be persisting the
toString() representation.

* Add DB and formatting changes

* Add tests, make minor fixes

db/src/main/resources/sql/flyway/V9__premium_list_currency_type.sql

@@ -0,0 +1,19 @@
+-- Copyright 2019 The Nomulus Authors. All Rights Reserved.
+--
+-- Licensed under the Apache License, Version 2.0 (the "License");
+-- you may not use this file except in compliance with the License.
+-- You may obtain a copy of the License at
+--
+--     http://www.apache.org/licenses/LICENSE-2.0
+--
+-- Unless required by applicable law or agreed to in writing, software
+-- distributed under the License is distributed on an "AS IS" BASIS,
+-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+-- See the License for the specific language governing permissions and
+-- limitations under the License.
+
+-- Note: We're OK with dropping this data since it's not live in production yet.
+alter table "PremiumList" drop column if exists currency;
+
+-- TODO(mcilwain): Add a subsequent schema change to remove this default.
+alter table "PremiumList" add column currency text not null default 'USD';