Add basic AllocationToken entity and tests

Logic actually using this entity to follow in subsequent CLs introducing
the command to generate/save these entities as well as the flow logic for
considering them during domain EPP operations.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181342600

javatests/google/registry/model/domain/AllocationTokenTest.java

@@ -0,0 +1,44 @@
+// Copyright 2017 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.
+
+package google.registry.model.domain;
+
+import static com.google.common.truth.Truth.assertThat;
+import static google.registry.model.ofy.ObjectifyService.ofy;
+import static google.registry.testing.DatastoreHelper.persistResource;
+
+import com.googlecode.objectify.Key;
+import google.registry.model.EntityTestCase;
+import google.registry.model.reporting.HistoryEntry;
+import org.junit.Test;
+
+/** Unit tests for {@link AllocationToken}. */
+public class AllocationTokenTest extends EntityTestCase {
+
+  @Test
+  public void testPersistence() throws Exception {
+    AllocationToken token =
+        persistResource(
+            new AllocationToken.Builder()
+                .setToken("abc123")
+                .setRedemptionHistoryEntry(Key.create(HistoryEntry.class, 1L))
+                .build());
+    assertThat(ofy().load().entity(token).now()).isEqualTo(token);
+  }
+
+  @Test
+  public void testIndexing() throws Exception {
+    verifyIndexing(new AllocationToken.Builder().setToken("abc123").build(), "token");
+  }
+}