API: Added system read endpoint

Standardised logic for reading app version to its own static class.

app/App/SystemApiController.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace BookStack\App;
+
+use BookStack\Http\ApiController;
+use Illuminate\Http\JsonResponse;
+
+class SystemApiController extends ApiController
+{
+    /**
+     * Read details regarding the BookStack instance.
+     * Some details may be null where not set, like the app logo for example.
+     */
+    public function read(): JsonResponse
+    {
+        $logoSetting = setting('app-logo', '');
+        if ($logoSetting === 'none') {
+            $logo = null;
+        } else {
+            $logo = $logoSetting ? url($logoSetting) : url('/logo.png');
+        }
+
+        return response()->json([
+            'version' => AppVersion::get(),
+            'instance_id' => setting('instance-id'),
+            'app_name' => setting('app-name'),
+            'app_logo' => $logo,
+            'base_url' => url('/'),
+        ]);
+    }
+}