Allow UploadFile widget dynamic links

[dlee1579]

I am currently using the UploadFile widget to manage file uploads to S3. I am saving only the file's location in S3 (e.g. s3://bucket_name/key_name) but to view the actual file in S3 I would have to generate a presigned url using boto3's generate_presigned_url() due to AWS resource permissioning.

I have used this pattern before in conjunction with the @display decorator for individual model admins. Can the file upload widget be modified to allow another override on the uploaded file so that the displayed upload link is dynamic?

[vsdudakov]

#130

[vsdudakov]

Fixed here https://github.com/vsdudakov/fastadmin/releases/tag/0.4.9