Add command controller to generate Thumbnail via API for existing files

[markusguenther]

If you have already assets of the type docx, ai or whatever type you want to use with filepreviews.io the files will have a thumbnails with a static resource. The used thumbnail is very often just have the icon for the mimetype.

The command controller can now go thru these files and request new thumbnails from the API and replace the static resource. Important is that we have the possibility to limit the amount of files because every filepreviews.io user has a monthly quota limit.

[markusguenther]

In the command controller we could also request the thumbnail in the biggest configured preset size and create the smaller thumbnails on our server as we have an image. That saves requests in the API quota.