username and provider are unique (username is no longer unique)

Some context for this change:

I went ahead and set up all the different providers as stubbed out in /server/config/env/development.js . Some providers (Google and LinkedIn) use my email address as my username. This caused an error because the User schema wanted the username to be unique. This restriction makes sense within the context of a provider, so I set the User schema to use a compound index on username and provider which is unique.

I hope this helps.

Cheers,
Mike