[feature] Add a device name changed signal, connect it to refresh the notifications cache

[nemesifier]

Blocked by openwisp/openwisp-notifications#164.
We should first confirm that openwisp/openwisp-notifications#164 can be done and then we can move on with this.

Like #241 but emitted when the name is changed.
PS: there's already a few lines of code which find out if the name has changed:

openwisp-controller/openwisp_controller/config/base/device.py

Lines 114 to 115 in a627b61

	if self.name != current.name and self._has_config():
	self.config.set_status_modified()

We can change the implementation to be like #241, which doesn't need an extra DB query which could be good to optimize the system a bit.

Addendum

I want to make clear what the issue is: right now the issue is that when the name of a device is changed, the notifications keep referring to it with the old name.

When a new device registers, it registers using the mac address as the name, then an operator usually changes its name to something more human readable, but the notification keep referring to it with the mac address.

[nemesifier]

PS: there's already a few lines of code which find out if the name has changed:

openwisp-controller/openwisp_controller/config/base/device.py

Lines 114 to 115 in a627b61

	if self.name != current.name and self._has_config():
	self.config.set_status_modified()

We can change the implementation to be like #241, which doesn't need an extra DB query which could be good to optimize the system a bit.

[nemesifier]

This is ready to be worked on, depends on openwisp/openwisp-notifications#168 which is ready, so that branch can be installed by editing the requirements.txt this way:

openwisp-notifications @ https://github.com/niteshsinha17/openwisp-notifications/tarball/issues/164-introduce-mechanism-to-clear-cache

[codesankalp]

I would like to work on it.