Replace oidc.domain_map with email to namespace

[kradalby]

Depends on the release of #320 and #311.

When we have removed the barriers between namespaces (soon to be called users), we should replace the configuration option oidc.domain_map with logic that just maps the username/email from OIDC straight as the namespace.

This will simplify the setup quite a bit and was the originally suggested setup, but @kradalby did not understand namespaces == users at the time and made a different suggestion.