Worker: missing easy deactivation action

It should be possible to quickly deactivate a number of workers from the configuration, w/o removing all of the configuration from its place. This would allow us to adjust the farm size on demand.

Items to be executed:

• Add a field on each worker indicating its state (active/deactive)
• Add an action to the worker admin to deactivate/activate workers in batch
• Modify the configuration logic so that, if a worker is active, is the same as it not belonging to the configuration
• Display this information adequately on the scheduler/backend page