Add PackageUpload task

The hardest bit of this was writing useful tests. I decided it would be best to have something resembling real FTP and SFTP servers in unit tests, so wrote basic ones using pyftpdlib and paramiko respectively.

While the design called for a server test, I concluded that it would be better to put it on a worker for resilience: if an upload hangs for some reason, then it will only take out one external worker, not one of the much more limited pool of Celery workers. The main point of that part of the design was to avoid needing an environment to run dput, which is still fulfilled here: we use ftplib or fabric to perform the uploads.

There are some new dependencies here, but they're quite light: around 2 MiB of installed size.

Part of #364 (closed).