Use composite primary key for RegistrarContact (#761)

* Use composite primary key for RegistrarPoc

* Increase the serial number for flyway file and resolve comments

* Rebase on HEAD

* Rebase on HEAD

db/src/main/resources/sql/flyway/V50__use_composite_key_for_registrar_poc.sql

@@ -0,0 +1,20 @@
+-- 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 "RegistrarPoc" add column registrar_id text;
+
+alter table "RegistrarPoc" drop constraint "RegistrarPoc_pkey";
+
+alter table "RegistrarPoc"
+    add constraint "RegistrarPoc_pkey" primary key (registrar_id, email_address);