[feature] Added support for device deactivation #560

[pandafy]

Closes #560

Blockers

• [feature] Implemented device deactivation and reactivation #625  openwisp-controller#840

[nemesifier]

What happens to the monitoring status of a device when it gets deactivated?
I was expecting to see some logic which would see some logic for the following:

• Making sure monitoring checks are not run
• Setting monitoring status to "UNKNOWN". Or, we can use a special status for this e,g, "DEACTIVATED" - I think "DEACTIVATED" is more explicit and preferable for UX
• Replying with 404 for monitoring requests

I would also add that activating a device must reverse this: checks are run again, hence monitoring status should automatically become either OK or CRITICAL and device metrics shall be accepted.

We need tu update the docs to reflect these changes.

Please rebase on the latest master and reformat code if necessary.