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 :sob:)
• [ ] 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

[ryantheleach]

This is too complicated to be completed before Launch, and probably requires external help and coordination. It's been pushed back to 1.7.0 by request of staff.

[progwml6]

it doesn't require external help, just support, and coordination to deal w/ some complexities around setting Fastly for ore to work with these changes without breaking things (when this config option for ore is enabled)