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/ConfigModule.java

@@ -395,6 +395,12 @@ public final class ConfigModule {
     }
   }
 
+  @Provides
+  @Config("registrarConsoleEnabled")
+  public static boolean provideRegistrarConsoleEnabled() {
+    return true;
+  }
+
   /** Maximum amount of time for syncing a spreadsheet, before killing. */
   @Provides
   @Config("sheetLockTimeout")