continuing from https://github.com/bbc/digital-paper-edit-client/issues/35 - [ ] end point API to retrieve signed urls from S3 - [ ] define S3 file structure - [ ] investigate if need to be able to determine APIs that would allow to get signed url for specific directories eg `/previews` etc.. see [S3 docs](https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/S3.html) and [this blog post](https://medium.com/@khelif96/uploading-files-from-a-react-app-to-aws-s3-the-right-way-541dd6be689)
continuing from bbc/digital-paper-edit-client#35
/previewsetc..see S3 docs and this blog post