Use non-password credentials for uploading

[gracenotes]

For example, an SSH key. This is a long-term goal, unclear exactly how to do it correctly.

[dcoutts]

Or GPG signed packages.

For example, if we supported optional package signing then we'd need a system like this:

• opt-in to requiring signing
• all future uploads would require a signature
• signature checked by the server (for policy purposes) and optionally by end users (for actual security)
• need a per-package keychain of authorised users (ie gpg keys for the members of the maintainer group)

[agrafix]

Or a simple API Key would be very useful.

[gbaz]

I think this was resolved by #534 although we can do better in the future...