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.

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

docs/flows.md

@@ -332,6 +332,7 @@ new ones with the correct approval time).
     *   The requested domain name is on the premium price list, and this
         registrar has blocked premium registrations.
 *   2306
+    *   Domain transfer period must be one year.
     *   Periods for domain registrations must be specified in years.
     *   The requested fees cannot be provided in the requested currency.