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

java/com/google/domain/registry/module/frontend/BUILD

@@ -38,6 +38,6 @@ java_binary(
         ":frontend",
         "//java/com/google/domain/registry/monitoring/whitebox",  # MetricsTaskServlet
         "//java/com/google/domain/registry/ui/server/api",  # CheckApiServlet
-        "//java/com/google/domain/registry/ui/server/registrar",  # ConsoleUiServlet, etc.
+        "//java/com/google/domain/registry/ui/server/registrar",  # ResourceServlet
     ],
 )