auth: ensure that userId and user name are unique among team members

[michaelkamphausen]

I added validations to check if the userId and user name are unique when a new member is admitted. The validation fails if another team member already uses the same userId or (case-insensitive) userName. I hope it helps with #72. I confess that I haven't thought about the tricky part of concurrency though.

[HerbCaudill]

Thanks, Michael! I do want to think through the concurrency issue so we can handle conflicts in a principled way, but this is an improvement over the status quo so I'll go ahead and merge it.