Support worker type model

Refs https://gitlab.com/arkindex/requests/-/issues/628

We need a new model dataimport.WorkerType with fields:

• UUID PK
• unique slug
• charfield display name
• created/update dates

Workers would have a required foreign key toward that type. A data migration would create each worker type needed (using unique slug), and link existing workers towards these types.

New workers (created through the API) should either get or create a new worker type.

Only admins would be able to manage the worker types through the admins (please declare them, listing them with slug, display name and creation date)