Replace JWT with agent tokens

https://redmine.teklia.com/issues/10050

• Drop the djangorestframework-simplejwt dependency, which we had in CE but only used in EE
• Remove the jwt_signing_key setting from arkindex.project.config
• Remove SIMPLE_JWT from the Django settings
• Rename arkindex.ponos.models.task_token_default to token_default
• Add an Agent.token field, exactly like Task.token
• Add a unique_agent_token unique constraint, just like unique_task_token but for Agent.token