Add support for reserved usernames

[mikkonie]

There is a use case in SODAR for having users logging in with a dummy username, which does not correspond to an actual user. It would make sense to prevent the creation of such user to avoid confusion.

There should be a Django setting with a list of user names not allowed by the system. On the creation of a SODARUser object, we would check against the list and refuse creating the object if the username is on this list.

TBD: Do this for local users only? I can't really figure out a case where this would be relevant for LDAP or OIDC users, unless someone has configured something horribly wrong.

[mikkonie]

The use case mentioned in SODAR is no longer valid, at least at this point. However, I'm leaving this issue open as it may well be required by something in the future.