File upload offloading

[lukegb]

Once uploaded, files should be kept locally for a short time before being offloaded to repo.spongepowered.org.

This means you need:

• small background task which waits for new files and periodically attempts to upload them over FTP (make background tasks separate to the webserver, please, I don't want multiple instances of webserver == dozens of copies of background task, that makes me 😭)
• ability to redirect to signed URL (signing algorithm TBD, it'll just be Yet Another Secret + timestamp)
• need to be able to differentiate between currently-locally-stored files and uploaded-to-repo files (if you want to serve immediately - alternate approach is that you just bite the bullet and don't display it to people until it's been uploaded, which works equally well)

[lol768]

This blocks #188