Add support for release channels (insider release channel) in Updater

app/Http/Controllers/AppVersionController.php

@@ -10,14 +10,21 @@ class AppVersionController extends Controller
     /**
      * Handle the incoming request.
      *
-     * @return \Illuminate\Http\Response
+     * @return \Illuminate\Http\JsonResponse
      */
     public function __invoke(Request $request)
     {
         $version = Setting::getSetting('version');
 
+        $channel = Setting::getSetting('updater_channel');
+        if (is_null($channel)) {
+            $channel = 'stable';
+            Setting::setSetting('updater_channel', 'stable'); // default.
+        }
+
         return response()->json([
             'version' => $version,
+            'channel' => $channel,
         ]);
     }
 }