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/RegistryConfig.java

@@ -201,11 +201,6 @@ public interface RegistryConfig {
    */
   public URL getRegistrarDefaultReferralUrl();
 
-  /**
-   * Returns whether the registrar console is enabled.
-   */
-  public boolean isRegistrarConsoleEnabled();
-
   /**
    * Returns the title of the project used in generating documentation.
    */