OAT will not load, if the nickname of another extid is identical to the current user (409 Conflict)

[oellers]

Current behaviour
At OAT startup, the route PUT extended-annotations/users returns a 409 conflict status for individual users, so loading never finishes

Suspected cause:

• User has an identical nickname (e.g. real name) but a different extid (xannotations_user) as another user who has previously used the OAT. 

Expected behaviour

• email and nickames are not necessarily unique, so this should not be an issue
• all columns should not be forced unique (except id and extid)

Env
Tested with a modified questionnaire branch version with a similar commit compared to the master branch; suspected:
1e6372c; should be fixed in backend