Implement "premium price ack required" checkbox

Implement a checkbox in the "Resources" tab to allow registrars to toggle
their "premium price ack required" flag.

Tested:
  Verfied the console functionality by hand.  I've started work on an
  automated test, but we can't actually test those from blaze and the
  kokoro tests are way too time-consuming to be practical for development, so
  we're going to have to either find a way to run those locally outside of
  the normal process or make do without a test.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=190212177

java/google/registry/model/registrar/Registrar.java

@@ -615,6 +615,7 @@ public class Registrar extends ImmutableObject implements Buildable, Jsonifiable
         .put("emailAddress", emailAddress)
         .put("whoisServer", getWhoisServer())
         .put("blockPremiumNames", blockPremiumNames)
+        .put("premiumPriceAckRequired", premiumPriceAckRequired)
         .put("url", url)
         .put("referralUrl", getReferralUrl())
         .put("icannReferralEmail", getIcannReferralEmail())