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/Api/ApiController.php

@@ -24,9 +24,14 @@ abstract class ApiController extends Controller
 
     /**
      * Get the validation rules for this controller.
+     * Defaults to a $rules property but can be a rules() method.
      */
     public function getValdationRules(): array
     {
+        if (method_exists($this, 'rules')) {
+            return $this->rules();
+        }
+
         return $this->rules;
     }
 }