Restrict domain transfer periods to 1 year

It turns out that this ICANN policy appears to prohibit transfers with
registration extensions other than 1 year (section A.8):
https://www.icann.org/resources/pages/policy-2012-03-07-en

This is backed up by the practical fact that we've never seen a registrar
request a transfer for any period other than one year.

And removing the support for multi-year transfers vastly simplifies
transfer logic and eliminates a bunch of annoying corner cases.  Users
still can achieve the same thing by doing a 1-year transfer plus a
manual renewal afterwards for the remainder of the desired extension.

This change leaves in place lots of infrastructure to support multi-year
transfers that is now obsolete (e.g. TransferData.extendedRegistrationYears).
This should all be cleaned up, but it's a lower priority than fixing the
gap itself and insulating ourselves against needing to handle any real
multi-year transfer case.  Once this CL goes in, we can start ignoring
extendedRegistrationYears entirely because it'll always be 1 year, which
makes the cleanup process easier.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=150212864

java/google/registry/flows/exceptions/TransferPeriodMustBeOneYearException.java

@@ -0,0 +1,24 @@
+// 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.flows.exceptions;
+
+import google.registry.flows.EppException.ParameterValuePolicyErrorException;
+
+/** Domain transfer period must be one year. */
+public class TransferPeriodMustBeOneYearException extends ParameterValuePolicyErrorException {
+  public TransferPeriodMustBeOneYearException() {
+    super("Domain transfer period must be one year");
+  }
+}