[feature] Allow to flag devices as inactive

[nemesifier]

Depends on openwisp/openwisp-controller#625.

We need to have a way to disable the monitoring checks on a device which is kept as inventory but not in use anymore.
We could add an admin action which allows to select the devices and flag them as inactive from the monitoring system's perspective.
We'd need a new status for this which would be similar to "unknown", unknown being the first status a device gets, while "inactive" would have to be manually set by admins. If a device gives any life sign it will automatically change status from inactive to ok.

It's almos as if we want to move a device back to "unknown" status, althogh unknown doesn't make sense here and it would be better to call this "inactive", "disabled" or something similar.

When flagging a device is inactive, we shall automatically flag its checks as is_active=False.
The periodic monitoring checks shall not be performed on inactive devices.

If the system detects any activity (collection of monitoring information, config checksum requests) on an inactive device it must enable the checks again (it will be better to perform these actions in a background celery task), this will allow an inactive device to become active again if needed.

This feature will have to be documented in the README.