Fix loading projects with custom extensions

Specifically, if someone saves a project with a custom extension in it, they will actually be able to load it if they go back to TW with that extension enabled.
We're not automatically loading custom extensions from projects, that's unsafe.

src/extension-support/extension-manager.js

@@ -208,6 +208,7 @@ class ExtensionManager {
      */
     registerExtensionService (serviceName) {
         dispatch.call(serviceName, 'getInfo').then(info => {
+            this._loadedExtensions.set(info.id, serviceName);
             this._registerExtensionInfo(serviceName, info);
         });
     }