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.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117969638

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

@@ -69,8 +69,7 @@ public final class RegistryTestServer {
           com.google.domain.registry.module.backend.BackendServlet.class),
 
       // Registrar Console
-      route("/registrar",
-          com.google.domain.registry.ui.server.registrar.ConsoleUiServlet.class),
+      route("/registrar", com.google.domain.registry.module.frontend.FrontendServlet.class),
       route("/registrar-settings",
           com.google.domain.registry.ui.server.registrar.RegistrarServlet.class),
       route("/registrar-payment",