monitoring/commands: Add `InstanceCommandForm'

The `InstanceCommandForm' should be the base class for forms that handle program-wide commands.

refs #6593
monitoring/commands: Add `ToggleFeatureCommandForm'

`ToggleFeatureCommandForm' should be the base class for forms enabling/disabling features of an Icinga instance.

refs #6593
monitoring/commands: Add (START|STOP)_EXECUTING_HOST_CHECKS commands

`ToggleActiveHostChecks' is the command object for enableing/disabling active host checks on an Icinga instance.

refs #6593
monitoring/commands: Add (START|STOP)_EXECUTING_SVC_CHECKS commands

`ToggleActiveServiceChecks' is the command object for enabling/disabling active service checks on an Icinga instance.

refs #6593
monitoring/commands: Add (ENABLE|DISABLE)_EVENT_HANDLERS commands

`ToggleEventHandlers' is the command object for enabling/disabling host and service event handlers on an Icinga instance.

refs #6593
monitoring/commands: Add (ENABLE|DISABLE)_FLAP_DETECTION commands

`ToggleFlapDetection' is the command object for enabling/disabling host and service flap detection on an Icinga instance.

refs #6593
monitoring/commands: Add (ENABLE|DISABLE)_NOTIFICATIONS/DISABLE_NOTIFICATIONS_EXPIRE_TIME commands

`ToggleNotifications' is the command object for enabling/disabling host and service notifications on an Icinga instance with an optional expire time for disabling notifications.

refs #6593
monitoring/commands: Add (START|STOP)_OBSESSING_OVER_HOST_CHECKS commands

`ToggleObsessingOverHostChecks' is the command object for enabling/disabling processing of host checks via the OCHP command on an Icinga instance.

refs #6593
monitoring/commands: Add (START|STOP)_OBSESSING_OVER_SERVICE_CHECKS commands

`ToggleObsessingOverServiceChecks' is the command object for enabling/disabling processing of service checks via the OCHP command on an Icinga instance.

refs #6593
monitoring/commands: Add (ENABLE|DISABLE)_PASSIVE_HOST_CHECKS commands

`TogglePassiveHostChecks' is the command object for enabling/disabling passive host checks on an Icinga instance.

refs #6593
monitoring/commands: Add (ENABLE|DISABLE)_PASSIVE_SERVICE_CHECKS commands

`TogglePassiveServiceChecks' is the command object for enabling/disabling passive service checks on an Icinga instance.

refs #6593
monitoring/commands: Add (ENABLE|DISABLE)_PERFORMANCE_DATA commands

`TogglePerformanceData' is the command object for enabling/disabling the processing of host and service performance data on an Icinga instance.

refs #6593