BadgeApp: User ids?

[david-a-wheeler]

We probably ought to talk about what is a user id.

For non-GitHub users, we could just use email addresses (Heroku does this). If we do, though, we ought to not display those usernames to the general public... perhaps they should only be displayed when you're viewing a project that you can also edit. We don't have to use email addresses, I just raise that as a possibility.

For GitHub users I don't see a problem displaying them, they're public anyway.

We'll need to make sure that the naming scheme is general, so that we can add systems other than GitHub in the future.