Added protections against path traversal in file system operations

- Files within the storage/ path could be accessed via path traversal
  references in content, accessed upon HTML export.
- This addresses this via two layers:
  - Scoped local flysystem filesystems down to the specific image &
    file folders since flysystem has built-in checking against the
    escaping of the root folder.
  - Added path normalization before enforcement of uploads/{images,file}
    prefix to prevent traversal at a path level.

Thanks to @Haxatron via huntr.dev for discovery and reporting.
Ref: https://huntr.dev/bounties/ac268a17-72b5-446f-a09a-9945ef58607a/

app/Config/filesystems.php

@@ -37,9 +37,14 @@ return [
             'root'   => public_path(),
         ],
 
-        'local_secure' => [
+        'local_secure_attachments' => [
             'driver' => 'local',
-            'root'   => storage_path(),
+            'root'   => storage_path('uploads/files/'),
+        ],
+
+        'local_secure_images' => [
+            'driver' => 'local',
+            'root'   => storage_path('uploads/images/'),
         ],
 
         's3' => [