API: Added system read endpoint

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

app/Settings/SettingController.php

@@ -3,6 +3,7 @@
 namespace BookStack\Settings;
 
 use BookStack\Activity\ActivityType;
+use BookStack\App\AppVersion;
 use BookStack\Http\Controller;
 use BookStack\Users\Models\User;
 use Illuminate\Http\Request;
@@ -26,12 +27,9 @@ class SettingController extends Controller
         $this->checkPermission('settings-manage');
         $this->setPageTitle(trans('settings.settings'));
 
-        // Get application version
-        $version = trim(file_get_contents(base_path('version')));
-
         return view('settings.categories.' . $category, [
             'category'  => $category,
-            'version'   => $version,
+            'version'   => AppVersion::get(),
             'guestUser' => User::getGuest(),
         ]);
     }