Create a worker without a repository

Depends #1624 (closed)
Refs https://redmine.teklia.com/issues/4471

An API user should be able to create a worker (no rights needed, just a verified email) using CreateWorker endpoint.

No changes on the serializer are needed, all 3 fields remain valid. But the authentication must support:

• ponos task auth on a repository process (like currently), which will create a worker WITH a repository
• user authentication which will create a worker WITHOUT a repository

The user that creates a worker is immediately assigned as administrator.