hub: utility for populating user records?

[oliver-sanders]

Jupyter Hub allows us to delegate the permissions for one user to start another users server.

This is great, however, for this functionality to work, the user who's server is being spawned must first log into the Hub. This creates an entry in the DB for them which is presumably required in order for another user to spawn their server?

This is inconvenient, but manageable in most cases. Users must log in to set up the hub, after that no further interaction is required until the hub is restarted?

However, in some cases we may have user accounts which can't be directly logged into. In these cases we need to initialise the hub to spawn servers some other way.

Related to #275