Thumbnails don't go through File Storage Locations

[Mnkras]

When a thumbnail is generated it doesn't go through file storage locations, it simply ends up in the cache directory,

This means that for sites that span multiple servers, it breaks thumbnails.

We need to route thumbnails through file storage locations so that everything doesn't break.

To be more specific, its the basic thumbnailer that has the issue, (https://github.com/concrete5/concrete5/blob/develop/web/concrete/src/File/Image/BasicThumbnailer.php)

I believe all the changes could be consolidated in there.

[Mnkras]

Upon further investigation, thumbnails can exist for deleted files which can also be seen as a bug, deleting a file should also remove all thumbnails.

[hissy]

Related: #1618

[Mnkras]

I made a quick patch that works to write and retrieve thumbnails from the default storage location, I tested this with local storage and my S3 storage addon:
https://gist.github.com/Mnkras/3949a5fb90c289e994f3