Prevent different users from using the same name

[PizieDust]

Currently multiple accounts can have the exact same name. We require the email to be unique but not the name.
This can be problematic given that the domain used by albatross is the name of the account.

[PizieDust]

What about using the users uuid for domain purposes? This can be a quick fix in this case.
But I assume the other frontends (grafana), may not like the uuid values (if it's displayed somewhere in the UI?)

[hannesm]

For now, esp. since we've a running albatross with existing setup (and there's no UUID prefix for the unikernels), and the question about where such a mapping (user name and uuid) should be persisted (if needed) -- see robur-coop/albatross#173 -- I'd prefer to check that the user name is unique.

[PizieDust]

Thank you for the additional context on the albatross issue. I totally agree it's a bit more complex to decide. I think we can easily just check the names and prevent duplicate ones for the time being.