Direct upload support

[batonac]

Currently local server file storage is used as the immediate upload location, upload_file pushes from the server to the s3 storage.

Alternatively, generate_presigned_url can be utilized to generate a put_object URL which can be used to upload a file from a client to the object storage directly, bypassing server storage entirely.

[agritheory]

@fproldan We'll have to handle a case where the File record isn't created because the upload fails or is cancelled in some way. I think the mock S3 endpoint test library will support something like this.