Pay off technical debt with ConsoleUiServlet

1. Turn ConsoleUiServlet into an action
2. Remove AbstractUiServlet, which fixes its threading bug
3. Use type-safe soy template parameters when rendering console

A follow-up change will add a new template parameter that renders the
payment page link on the navigation bar.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117969638

java/com/google/domain/registry/config/TestRegistryConfig.java

@@ -149,11 +149,6 @@ public class TestRegistryConfig implements RegistryConfig {
     return makeUrl("http://www.referral.example/path");
   }
 
-  @Override
-  public boolean isRegistrarConsoleEnabled() {
-    return true;
-  }
-
   @Override
   public String getDocumentationProjectTitle() {
     return "Domain Registry";