Redirect Registrar.referralUrl UI actions to url field

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=196597051

javatests/google/registry/testing/AppEngineRule.java

@@ -165,6 +165,7 @@ public final class AppEngineRule extends ExternalResource {
         .setState(State.ACTIVE)
         .setEmailAddress("new.registrar@example.com")
         .setIcannReferralEmail("lol@sloth.test")
+        .setUrl("http://my.fake.url")
         .setInternationalizedAddress(new RegistrarAddress.Builder()
             .setStreet(ImmutableList.of("123 Example Boulevard"))
             .setCity("Williamsburg")