Added an env configurable file upload size limit

Replaces the old suggestion of setting JS head 'window.uploadLimit'
variable. This new env option will be used by back-end validation and
front-end libs/logic too.

Limits already likely exist within prod environments at a PHP and
webserver level but this allows an app-level limit and centralises the
option on the BookStack side into the .env

Closes #3033

app/Http/Controllers/Controller.php

@@ -165,7 +165,7 @@ abstract class Controller extends BaseController
     /**
      * Log an activity in the system.
      *
-     * @param $detail string|Loggable
+     * @param string|Loggable $detail
      */
     protected function logActivity(string $type, $detail = ''): void
     {
@@ -177,6 +177,6 @@ abstract class Controller extends BaseController
      */
     protected function getImageValidationRules(): array
     {
-        return ['image_extension', 'mimes:jpeg,png,gif,webp'];
+        return ['image_extension', 'mimes:jpeg,png,gif,webp', 'max:' . (config('app.upload_limit') * 1000)];
     }
 }