[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.

[nemesifier]

Color

Let's make the color of deactivate status black (#000), both in the health status (which is now of the same color of unknown) and in the pie chart:

Device details: hide checks

Once the device is deactivated, I think we shouldn't show the checks in the device details anymore:

Cache invalidation

Let's ensure that once the device is deactivated, the monitoring API responds 404 correctly, during testing we noticed a cache invalidation issue.