Clean up flow validation of registration periods

This tidies up some logic in the flows that checks registration periods, so that in the create flows we're consistently checking that the requested number of years is <= 10 right away (DomainCreateFlow was deferring it until very late, including after custom logic ran, for no good reason I can see).

It also refactors the validateRegistrationPeriod() overload used by DomainRenewFlow to take the newExpirationTime directly, and just check to ensure that it's >= to now.plusYears(10) (with leap-safety just in case).  This is a much simpler check than before, which recomputed the newExpirationTime separately from the logic used by DomainRenewFlow itself (always dangerous) and did a more convoluted and unnecessary comparison involving extendRegistrationWithCap().

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=151002960

java/google/registry/flows/domain/DomainCreateFlow.java

@@ -190,6 +190,7 @@ public class DomainCreateFlow implements TransactionalFlow {
     Period period = command.getPeriod();
     verifyUnitIsYears(period);
     int years = period.getValue();
+    validateRegistrationPeriod(years);
     failfastForCreate(targetId, now);
     verifyResourceDoesNotExist(DomainResource.class, targetId, now);
     // Validate that this is actually a legal domain name on a TLD that the registrar has access to.
@@ -248,7 +249,6 @@ public class DomainCreateFlow implements TransactionalFlow {
         validateSecDnsExtension(eppInput.getSingleExtension(SecDnsCreateExtension.class));
     String repoId = createDomainRepoId(ObjectifyService.allocateId(), registry.getTldStr());
     DateTime registrationExpirationTime = leapSafeAddYears(now, years);
-    validateRegistrationPeriod(years);
     HistoryEntry historyEntry = buildHistory(repoId, period, now);
     // Bill for the create.
     BillingEvent.OneTime createBillingEvent =