MVP for Package registry API

[blaix]

• Register a new user, or login
• Register a publishing identity (gren-lang is a publishing identity, me@example.com is a user)
• Add or remove additional users to a publishing identity
• Permissions in a publishing identity (only download, publish new packages, user admin)
• Publish new packages/versions
• Revoke a package/version (the name/version is still registered, but the actual content is removed).
• Ask which version is latest for a given compiler version, and which was the previous version related to some version number.
• Misc TODOs in README and code.