Create a worker

Depends backend#1625 (closed)
Refs https://redmine.teklia.com/issues/4471

A user should be able to create a worker from the workers list (url /process/workers).

A new button on the right of "Available workers" must be added, trigerring a modal that allow a user to input all 3 fields required by CreateWorker:

• name
• type (no list, just text input)
• slug (auto created from name, but still editable)