Use Laravel Storage for file uploads

[nie7321]

When we originally implemented S3 file uploads, both Storage and Flysystem's S3 adapter made it hard to access the AWS SDK's S3Client with all the config set up, and these things did not have mechanisms to interact with S3-specific features.

This has been improved in Laravel 9.x and 10.x: the Storage::temporaryUrl() method can probably do everything we need, letting us simplify the file upload code significantly.