[feature] Emit signal when requests to the freeradius accounting view are processed successfully

[nemesifier]

Before a successful response is returned from the freeradius accounting view, we need to emit a signal so we can connect to it from other parts of the application or from third party apps.

The signal should not be emitted if there was any error.

The signal could be called radius_accounting_success, we should pass as argument the radius accounting object instance and the request object.

We shall add tests and docs for this signal. As for the docs, I think we'll need a new page called signals.rst in docs/source/developer, the signal should be documented as we document signals in other modules (eg: openwisp-controller).