Add an unrenew_domain command to nomulus tool

This is used to reduce the expiration time of domain(s) by some number of years
(if enough length remains in the registration term to do so). This does not back
out the previously saved BillingEvent entities as they may have already been
sent out and invoiced, so any related refunds must be handled out of band.

In addition to reducing the registration expiration time on the domain itself,
this command writes out a new history entry, one-time poll message informing the
registrar of this change, auto-renew billing event and poll message, and
updates/ends the old auto-renew billing event and poll message.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=224999285

java/google/registry/util/DateTimeUtils.java

@@ -77,4 +77,13 @@ public class DateTimeUtils {
     checkArgument(years >= 0);
     return years == 0 ? now : now.plusYears(1).plusYears(years - 1);
   }
+
+  /**
+   * Subtracts years from a date, in the {@code Duration} sense of semantic years. Use this instead
+   * of {@link DateTime#minusYears} to ensure that we never end up on February 29.
+   */
+  public static DateTime leapSafeSubtractYears(DateTime now, int years) {
+    checkArgument(years >= 0);
+    return years == 0 ? now : now.minusYears(1).minusYears(years - 1);
+  }
 }