Use the correct text VKey for HostResource's superordinateDomain (#608)

* Store the superordinateDomain reference as a VKey rather than Key

This is a reference to a Domain object, so we should store it as a VKey
in reference to the Domain table. This should not affect any business
logic, but rather will allow us to set up the SQL tables for
HostResource et al. properly.

db/src/main/resources/sql/flyway/V28__superordinate_domain_vkey.sql

@@ -0,0 +1,19 @@
+-- Copyright 2020 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.
+
+ALTER TABLE "HostResource" ALTER COLUMN superordinate_domain TYPE text;
+
+ALTER TABLE IF EXISTS "HostResource"
+   ADD CONSTRAINT fk_host_resource_superordinate_domain
+   FOREIGN KEY (superordinate_domain) REFERENCES "Domain"(repo_id);