Fix exception in bootExtensions

Frontend extenders exist in a weird state of limbo, where they are technically defined, but aren't used or tested at all. In da5db71, we shifted from passing `extension.extend` to `flattenDeep` to calling `flat` on `extension.extend`. If an extension doesn't define extenders (as is the case for most extensions), the change breaks the forum. All we do here is add a null check.

js/src/common/Application.js

@@ -179,10 +179,14 @@ export default class Application {
     this.initialRoute = window.location.href;
   }
 
+  // TODO: This entire system needs a do-over for v2
   bootExtensions(extensions) {
     Object.keys(extensions).forEach((name) => {
       const extension = extensions[name];
 
+      // If an extension doesn't define extenders, there's nothing more to do here.
+      if (!extension.extend) return;
+
       const extenders = extension.extend.flat(Infinity);
 
       for (const extender of extenders) {