Add payment page link to Registrar Console

The payment page has also been updated to refuse to show itself if
the registrar is not set to credit card billing terms.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117983313

javatests/com/google/domain/registry/server/Fixture.java

@@ -140,6 +140,7 @@ public enum Fixture {
       persistResource(
           Registrar.loadByClientId("TheRegistrar").asBuilder()
               .setAllowedTlds(ImmutableSet.of("example", "xn--q9jyb4c"))
+              .setBillingMethod(Registrar.BillingMethod.BRAINTREE)
               .build());
     }
   };