Save which agent is running a task. This is now visible in the admin UI
in the queue and in the agent details screen.
# changes
- [x] save id of agent executing a task
- [x] add endpoint to get tasks of an agent for #999
- [x] show assigned agent-id in queue
- [x] (offtopic) use same colors for queue stats and icons (similar to
the ones used by pipelines)
- [x] (offtopic) use badges for queue labels & dependencies
# Changes
- Adds an admin view to see the whole work-queue of the server.
- The admin can also pause / resume the queue.
- The view is reloading data every 5 seconds automatically.
- The task model from queue got removed in favor of the one from models.
